Comparison of SCSI, SAS and SATA interfaces. Hard drive connection interfaces: SCSI, SAS, Firewire, IDE, SATA sas controller what

Hard drive for a server, features of choice

The hard drive is the most valuable component in any computer. After all, it stores information that the computer and the user work with, if we are talking about a personal computer. Every time a person sits down at a computer, he expects that the operating system loading screen will now run through, and he will begin working with his data, which the hard drive will produce “on the mountain” from its depths. If we are talking about a hard drive, or even an array of them as part of a server, then there are tens, hundreds and thousands of users who expect to gain access to personal or work data. And all their quiet work or rest and entertainment depends on these devices, which constantly store data. Already from this comparison it is clear that the demands placed on home and industrial-class hard drives are unequal - in the first case, one user works with it, in the second - thousands. It turns out that the second hard drive should be many times more reliable, faster, and more stable than the first, because many users work with it and rely on it. This article will look at the types of hard drives used in the corporate sector and the features of their design that allow them to achieve the highest reliability and performance.

SAS and SATA drives - so similar and so different

Until recently, the standards of industrial-class and household hard drives differed significantly and were incompatible - SCSI and IDE, but now the situation has changed - the overwhelming majority of hard drives on the market are SATA and SAS (Serial Attached SCSI). The SAS connector is universal in form factor and is compatible with SATA. This allows you to directly connect to the SAS system both high-speed, but small-capacity (at the time of writing - up to 300 GB) SAS drives, and lower-speed, but many times more capacious, SATA drives (at the time of writing - up to 2 TB ). Thus, in one disk subsystem, you can combine mission-critical applications that require high performance and rapid access to data, and more economical applications with a lower cost per gigabyte.

This design compatibility benefits both back panel manufacturers and end users by reducing hardware and engineering costs.

That is, both SAS and SATA devices can be connected to SAS connectors, but only SATA devices can be connected to SATA connectors.

SAS and SATA - high speed and large capacity. What to choose?

SAS drives, which replaced SCSI drives, completely inherited their main properties that characterize a hard drive: spindle speed (15,000 rpm) and volume standards (36,74,147 and 300 GB). However, SAS technology itself is significantly different from SCSI. Let's briefly look at the main differences and features: The SAS interface uses a point-to-point connection - each device is connected to the controller by a dedicated channel, in contrast, SCSI operates over a common bus.

SAS supports a large number of devices (>16384), while SCSI supports 8, 16, or 32 devices per bus.

The SAS interface supports data transfer rates between devices at speeds of 1.5; 3; 6 Gb/s, while for the SCSI interface the bus speed is not allocated to each device, but is divided between them.

SAS supports connecting slower SATA devices.

SAS configurations are much easier to install and install. Such a system is easier to scale. In addition, SAS hard drives inherited the reliability of SCSI hard drives.

When choosing a disk subsystem - SAS or SATA, you need to be guided by what functions will be performed by the server or workstation. To do this, you need to decide on the following questions:

1. How many simultaneous diverse requests will the disk process? If it's large, your clear choice is SAS disks. Also, if your system will serve a large number of users, choose SAS.

2. How much information will be stored on the disk subsystem of your server or workstation? If it is more than 1-1.5 TB, you should pay attention to a system based on SATA hard drives.

3. What is the budget allocated for the purchase of a server or workstation? It should be remembered that in addition to SAS disks, you will need a SAS controller, which also needs to be taken into account.

4. Do you plan to subsequently increase the volume of data, increase productivity, or increase system fault tolerance? If yes, then you will need a SAS-based disk subsystem; it is easier to scale and more reliable.

5. Your server will work with critical data and applications - Your choice is SAS drives designed for harsh operating conditions.

A reliable disk subsystem includes not only high-quality hard drives from a renowned manufacturer, but also an external disk controller. They will be discussed in one of the following articles. Let's look at SATA drives, what types of these drives there are and which ones should be used when building server systems.

SATA drives: household and industrial sectors

SATA drives, used everywhere, from consumer electronics and home computers to high-performance workstations and servers, differ into subtypes, there are drives for use in household appliances, with low heat generation, power consumption, and, as a result, reduced performance, there are middle-class drives, for home computers, and there are drives for high-end systems. In this article we will look at the class of hard drives for high-performance systems and servers.

Performance characteristics

	Server class HDD	HDD desktop class
Rotational speed	7,200 rpm (nominal)	7,200 rpm (nominal)
Cache size
Average delay time	4.20 ms (nominal)	6.35 ms (nominal)
Data transfer rate
Reading from drive cache (Serial ATA)	maximum 3 Gb/s	maximum 3 Gb/s
physical characteristics
Capacity after formatting	1,000,204 MB	1,000,204 MB
Capacity
Interface	SATA 3 Gb/s	SATA 3 Gb/s
Number of sectors available to the user	1 953 525 168	1 953 525 168
Dimensions
Height	25.4 mm	25.4 mm
Length	147 mm	147 mm
Width	101.6 mm	101.6 mm
	0.69 kg	0.69 kg
Impact resistance
Impact resistance in working condition	65G, 2ms	30G; 2 ms
Impact resistance when not in use	250G, 2ms	250G, 2ms
Temperature
In working order	-0°C to 60°C	-0°C to 50°C
Inoperative	-40°C to 70°C	-40°C to 70°C
Humidity
In working order		relative humidity 5-95%
Inoperative	relative humidity 5-95%	relative humidity 5-95%
Vibration
In working order
Linear	20-300 Hz, 0.75 g (0 to peak)	22-330 Hz, 0.75 g (0 to peak)
free	0.004 g/Hz (10 - 300 Hz)	0.005 g/Hz (10 - 300 Hz)
Inoperative
Low frequency	0.05 g/Hz (10 - 300 Hz)	0.05 g/Hz (10 - 300 Hz)
High frequency		20-500 Hz, 4.0G (0 to peak)

The table shows the characteristics of hard drives from one of the leading manufacturers; one column shows data for a server-class SATA hard drive, and the other for a regular SATA hard drive.

From the table we see that disks differ not only in performance characteristics, but also in operational characteristics, which directly affect the life expectancy and successful operation of the hard drive. Please note that these hard drives differ only slightly in appearance. Let's look at what technologies and features allow us to do this:

The reinforced shaft (spindle) of the hard drive is fixed at both ends by some manufacturers, which reduces the influence of external vibration and facilitates precise positioning of the head unit during read and write operations.

The use of special intelligent technologies that take into account both linear and angular vibration, which reduces head positioning time and increases disk performance by up to 60%

The function of eliminating errors during operation in RAID arrays prevents hard drives from falling out of RAID, which is a characteristic feature of conventional hard drives.

Adjustment of the flight height of the heads in combination with technology to prevent contact with the surface of the platters, which leads to a significant increase in the life of the disk.

A wide range of self-diagnosis functions that allow you to predict in advance the moment when the hard drive fails and warn the user about it, which allows you to have time to save information to a backup drive.

Features that reduce the rate of unrecoverable read errors, which increases the reliability of the server hard drive compared to conventional hard drives.

Speaking about the practical side of the issue, we can confidently say that specialized hard drives in servers “behave” much better. There are significantly fewer calls to the technical service regarding instability of RAID arrays and hard drive failures. Manufacturer support for the server segment of hard drives occurs much more quickly than conventional hard drives, due to the fact that the priority area of ​​work for any manufacturer of data storage systems is the industrial sector. After all, it is here that the most advanced technologies are used to protect your information.

Analogue of SAS disks:

Hard drives from Western Digital VelociRaptor. These drives have a disk rotation speed of 10 thousand rpm, equipped with a SATA 6 Gb/s interface and 64 MB of cache memory. The time between failures of these drives is 1.4 million hours.
More details on the manufacturer's website www.wd.com

You can order the assembly of a server based on SAS or an analogue of SAS hard drives from our company "Status" in St. Petersburg; also, you can buy or order SAS hard drives in St. Petersburg:

• call +7-812-385-55-66 in St. Petersburg
• write to the address
• leave an application on our website on the "Online application" page

For more than 20 years, the parallel bus interface has been the most common communication protocol for most digital storage systems. But as the need for throughput and system flexibility has grown, the shortcomings of the two most common parallel interface technologies have become apparent: SCSI and ATA. The lack of compatibility between parallel SCSI and ATA interfaces—different connectors, cables, and command sets used—increases the cost of system maintenance, research and development, training, and qualification of new products.

To date, parallel technologies are still satisfactory for users of modern enterprise systems in terms of performance, but the growing demands for higher speeds, higher data security during transmission, reduction in physical size, as well as wider standardization call into question the ability of a parallel interface without unnecessary costs to keep pace with rapidly increasing CPU performance and hard drive speeds. In addition, in times of austerity, it is becoming increasingly difficult for enterprises to find funds for the development and maintenance of heterogeneous connectors on the back panels of server cases and external disk arrays, testing for compatibility of heterogeneous interfaces and inventory of heterogeneous connections for performing I/O operations.

The use of parallel interfaces also poses a number of other problems. Parallel data transmission over a wide daisy chain is subject to crosstalk, which can create additional interference and lead to signal errors - to avoid this trap, you have to reduce the signal speed or limit the cable length, or do both. Termination of parallel signals is also associated with certain difficulties - you have to terminate each line separately, usually this operation is performed by the last drive, in order to prevent the signal from being reflected at the end of the cable. Finally, the large cables and connectors used in parallel interfaces make these technologies unsuitable for new compact computing systems.

Introducing SAS and SATA

Serial technologies such as Serial ATA (SATA) and Serial Attached SCSI (SAS) overcome the architectural limitations of traditional parallel interfaces. These new technologies got their name from the method of signal transmission, when all information is transmitted sequentially (English serial), in a single stream, in contrast to multiple streams that are used in parallel technologies. The main advantage of a serial interface is that when data is transferred in a single stream, it moves much faster than when using a parallel interface.

Serial technologies combine many bits of data into packets and then transmit them over a cable at speeds up to 30 times faster than parallel interfaces.

SATA extends the capabilities of traditional ATA technology, allowing data transfer between disk drives at speeds of 1.5 GB per second and higher. Due to its low cost per gigabyte of disk capacity, SATA will remain the dominant disk interface in desktop PCs, entry-level servers, and network-attached storage systems where cost is a major consideration.

SAS technology, the successor to parallel SCSI, builds on the proven functionality of its predecessor and promises to significantly enhance the capabilities of today's enterprise storage systems. SAS offers a number of advantages that traditional storage solutions do not offer. In particular, SAS allows you to connect up to 16,256 devices to one port and provides a reliable point-to-point serial connection at speeds of up to 3 Gb/s.

Additionally, with a smaller connector, SAS provides full dual-port connectivity for both 3.5" and 2.5" drives (previously only available for 3.5" Fiber Channel drives). This is a very useful feature when you need to fit a large number of redundant drives into a compact system, such as a low-profile blade server.

SAS improves drive addressing and connectivity with hardware expanders that allow large numbers of drives to be connected to one or more host controllers. Each expander provides connection to up to 128 physical devices, which can be other host controllers, other SAS expanders or disk drives. This scheme scales well and allows you to create enterprise-scale topologies that easily support multi-node clustering for automatic system recovery in case of failure and for uniform load distribution.

One of the biggest benefits of the new serial technology is that the SAS interface will also be compatible with lower-cost SATA drives, allowing system designers to use both types of drives in the same system without incurring additional costs to support two different interfaces. Thus, SAS, the next generation of SCSI technology, overcomes the current limitations of parallel technologies in terms of performance, scalability and data availability.

Multiple levels of compatibility

Physical Compatibility

The SAS connector is universal and is compatible with SATA in form factor. This allows both SAS and SATA drives to be directly connected to the SAS system, allowing the system to be used either for mission-critical applications that require high performance and fast data access, or for more cost-effective applications with a lower cost per gigabyte.

The SATA command set is a subset of the SAS command set, allowing compatibility between SATA devices and SAS controllers. However, SAS drives cannot work with a SATA controller, so they are equipped with special keys on the connectors to eliminate the possibility of incorrect connection.

Additionally, the similar physics of the SAS and SATA interfaces allows for a new universal SAS backplane that supports both SAS and SATA drives. As a result, there is no need to use two different backplanes for SCSI and ATA drives. This design compatibility benefits both back panel manufacturers and end users by reducing hardware and engineering costs.

Protocol Compatibility

SAS technology includes three types of protocols, each of which is used to transfer different types of data over the serial interface depending on what device is being accessed. The first is the serial SCSI protocol (Serial SCSI Protocol SSP), transmitting SCSI commands, the second is the SCSI management protocol (SCSI Management Protocol SMP), transmitting control information to the expanders. The third, SATA Tunneled Protocol STP, establishes a connection that allows SATA commands to be transmitted. Thanks to the use of these three protocols, the SAS interface is fully compatible with existing SCSI applications, control software and SATA devices.

This multi-protocol architecture, combined with the physical compatibility of SAS and SATA connectors, makes SAS technology the universal link between SAS and SATA devices.

Benefits of Compatibility

SAS and SATA compatibility provides a number of benefits to system designers, builders, and end users.

System designers can use the same backplanes, connectors and cable connections thanks to SAS and SATA compatibility. Upgrading a system with the transition from SATA to SAS actually comes down to replacing disk drives. In contrast, for users of traditional parallel interfaces, moving from ATA to SCSI means replacing backplanes, connectors, cables, and drives. Other cost-effective benefits of consistent technology interoperability include simplified certification and asset management.

VAR resellers and system builders can easily and quickly reconfigure custom systems by simply installing the appropriate disk drive into the system. There is no need to work with incompatible technologies and use special connectors and different cable connections. Moreover, the added flexibility to balance price and performance will allow VAR resellers and system builders to better differentiate their products.

For end users, SATA and SAS compatibility means a new level of flexibility when it comes to choosing the best price/performance ratio. SATA drives will be the best solution for low-cost servers and storage systems, while SAS drives will provide maximum performance, reliability and compatibility with management software. The ability to upgrade from SATA drives to SAS drives without having to purchase a new system greatly simplifies the purchasing decision, protects your system investment, and reduces total cost of ownership.

Joint development of SAS and SATA protocols

On January 20, 2003, the SCSI Trade Association (STA) and the Serial ATA (SATA) II Working Group announced a collaboration to ensure system-level compatibility of SAS technology with SATA disk drives.

The collaboration between the two organizations, as well as the combined efforts of storage vendors and standards committees, aims to provide even more precise interoperability guidelines to help system designers, IT professionals, and end users fine-tune their systems for optimal performance. and reliability and lower total cost of ownership.

The SATA 1.0 specification was approved in 2001, and today there are SATA products on the market from various manufacturers. The SAS 1.0 specification was approved in early 2003, and the first products should hit the market in the first half of 2004.

With the advent of a sufficiently large number of Serial Attached SCSI (SAS) peripherals, we can state the beginning of the transition of the corporate environment to the new technology. But SAS is not only the recognized successor to UltraSCSI technology, but also realizes new areas of use, raising the scalability of systems to unimaginable heights. We decided to demonstrate the potential of SAS by taking a closer look at the technology, host adapters, hard drives and storage systems.

SAS isn't a completely new technology: it's the best of both worlds. The first part of SAS deals with serial data transmission, which requires less physical wires and pins. The transition from parallel to serial transmission made it possible to get rid of the bus. Although current SAS specifications specify throughput at 300 MB/s per port, which is less than UltraSCSI's 320 MB/s, replacing the shared bus with a point-to-point connection is a significant benefit. The second part of SAS is the SCSI protocol, which remains powerful and popular.

SAS can use a large set types of RAID. Giants such as Adaptec or LSI Logic offer an expanded set of functions in their products for expansion, migration, nesting and other capabilities, including those related to distributed RAID arrays across multiple controllers and drives.

Finally, most of the actions mentioned today are performed on the fly. Here we should mention excellent products AMCC/3Ware , Areca And Broadcom/Raidcore, allowing enterprise-class functions to be transferred to SATA spaces.

Compared to SATA, the traditional SCSI implementation loses ground on all fronts, with the exception of high-end enterprise solutions. SATA offers suitable hard drives, has a good price and a wide range solutions. And let's not forget about another smart feature of SAS: it easily coexists with existing SATA infrastructures, since SAS host adapters easily work with SATA drives. But you won’t be able to connect a SAS drive to a SATA adapter.

Source: Adaptec

First, it seems to us, we should turn to the history of SAS. The SCSI standard (stands for "small computer system interface") has always been considered as a professional bus for connecting drives and some other devices to computers. Hard drives for servers and workstations still use SCSI technology. Unlike the mainstream ATA standard, which allows you to connect only two drives to one port, SCSI allows you to connect up to 15 devices onto one bus and offers a powerful command protocol. Devices must have a unique SCSI ID, which can be assigned either manually or through the SCAM (SCSI Configuration Automatically) protocol. Since the device IDs for the buses of two or more SCSI adapters may not be unique, LUNs (Logical Unit Numbers) have been added to help identify devices in complex SCSI environments.

SCSI hardware is more flexible and reliable compared to ATA (this standard is also called IDE, Integrated Drive Electronics). Devices can be connected both inside the computer and outside, and the cable length can be up to 12 m, as long as it is properly terminated (in order to avoid signal reflections). As SCSI evolved, numerous standards emerged that specified different bus widths, clock frequencies, connectors and signal voltages (Fast, Wide, Ultra, Ultra Wide, Ultra2, Ultra2 Wide, Ultra3, Ultra320 SCSI). Fortunately, they all use the same set of commands.

Any SCSI communication is organized between the initiator (host adapter) sending commands and the target drive responding to them. Immediately after receiving a set of commands, the target drive sends a so-called sense code (status: busy, error or free), by which the initiator knows whether he will receive the desired response or not.

The SCSI protocol specifies almost 60 different commands. They are divided into four categories: non-data, bi-directional, read data, and write data.

The limitations of SCSI start to show themselves as you add drives to the bus. Today you can hardly find a hard drive that can fully load the 320 MB/s bandwidth of Ultra320 SCSI. But five or more drives on one bus is a completely different matter. An option would be to add a second host adapter for load balancing, but this costs money. The problem is with cables: twisted 80-wire cables are very expensive. If you also want to get a “hot replacement” of drives, that is, easy replacement of a failed drive, then special equipment (backplane) is required.

Of course, it is best to place the drives in separate snap-ins or modules, which are usually hot-swappable along with other nice control features. As a result, there are more professional SCSI solutions on the market. But they all cost a lot, which is why the SATA standard has developed so rapidly in recent years. Although SATA will never satisfy the needs of high-end enterprise systems, it is a great complement to SAS in creating new, scalable solutions for next-generation networking environments.

SAS does not share a bus across multiple devices. Source: Adaptec

SATA

On the left there is a SATA connector for data transfer. On the right is the power supply connector. There are enough pins to supply 3.3V, 5V and 12V to each SATA drive.

The SATA standard has been on the market for several years, and today it has reached its second generation. SATA I featured 1.5 Gbps throughput with two serial connections using low-voltage differential signaling. At the physical layer, 8/10 bit encoding is used (10 actual bits for 8 bits of data), which explains the maximum interface throughput of 150 MB/s. After the transition of SATA to a speed of 300 MB / s, many began to call the new standard SATA II, although when standardizing SATA-IO(International Organization) it was planned to first add more functions and then call it SATA II. Hence the latest specification is called SATA 2.5, it includes such SATA extensions as Native Command Queuing(NCQ) and eSATA (external SATA), port multipliers (up to four drives per port), etc. But additional SATA functions are optional for both the controller and the hard drive itself.

Let's hope that SATA III at 600 MB/s will still be released in 2007.

While parallel ATA (UltraATA) cables were limited to 46 cm, SATA cables can be up to 1 m long, and for eSATA twice that length. Instead of 40 or 80 wires, serial transmission requires only a few contacts. Therefore, SATA cables are very narrow, easy to route inside the computer case, and do not interfere with airflow as much. The SATA port relies on one device, which makes it a point-to-point interface.

SATA connectors for data transfer and power supply have separate plugs.

SAS

The signaling protocol here is the same as that of SATA. Source: Adaptec

A nice feature of Serial Attached SCSI is that the technology supports both SCSI and SATA, as a result of which SAS or SATA drives (or both standards at once) can be connected to SAS controllers. However, SAS drives cannot work with SATA controllers due to the use of the Serial SCSI Protocol (SSP). Like SATA, SAS follows a point-to-point connection principle for drives (300 MB/s today), and thanks to SAS expanders (or expanders), you can connect more drives than there are available SAS ports. SAS hard drives support two ports, each with its own unique SAS ID, so you can use two physical connections to provide redundancy by connecting the drive to two different host nodes. Thanks to the STP (SATA Tunneling Protocol), SAS controllers can exchange data with SATA drives connected to the expander.

Source: Adaptec

Source: Adaptec

Source: Adaptec

Of course, the only physical connection of the SAS expander to the host controller can be considered a “bottleneck”, so the standard provides wide SAS ports. A wide port groups multiple SAS connections into a single connection between any two SAS devices (typically between a host controller and an extender). The number of connections within the communication can be increased, it all depends on the requirements imposed. But redundant connections are not supported, nor are any loops or rings allowed.

Source: Adaptec

Future SAS implementations will add 600 and 1200 MB/s per port throughput. Of course, the performance of hard drives will not increase in the same proportion, but it will be more convenient to use expanders on a small number of ports.

Devices called "Fan Out" and "Edge" are expanders. But only the main Fan Out expander can handle the SAS domain (see 4x link in the center of the diagram). Each Edge expander allows up to 128 physical connections, and you can use wide ports and/or connect other expanders/drives. The topology can be quite complex, but at the same time flexible and powerful. Source: Adaptec

Source: Adaptec

The backplane is the basic building block of any storage system that must support hot plugging. Therefore, SAS expanders often imply powerful equipment (both in a single package and not). Typically, a single link is used to connect a simple device to a host adapter. Expanders with built-in accessories, of course, rely on multi-channel connections.

Three types of cables and connectors have been developed for SAS. SFF-8484 is a multi-core internal cable connecting the host adapter to the equipment. In principle, the same thing can be achieved by branching this cable at one end into several separate SAS connectors (see illustration below). SFF-8482 is a connector through which the drive is connected to a single SAS interface. Finally, the SFF-8470 is an external multi-core cable up to six meters long.

Source: Adaptec

Cable SFF-8470 for external multi-channel SAS connections.

Multicore cable SFF-8484. Four SAS channels/ports pass through one connector.

Cable SFF-8484, allowing you to connect four SATA drives.

SAS as part of SAN solutions

Why do we need all this information? Most users will not come close to the SAS topology we described above. But SAS is more than a next-generation interface for professional hard drives, although it is ideal for building simple and complex RAID arrays based on one or more RAID controllers. SAS is capable of more. This is a point-to-point serial interface that easily scales as you add more links between any two SAS devices. SAS drives come with two ports, so you can connect one port through an expander to a host system, and then create a backup path to another host system (or another expander).

Communication between SAS adapters and expanders (and between two expanders) can be as wide as there are SAS ports available. Expanders are usually rack-mount systems that can accommodate a large number of drives, and the possible connection of SAS to a higher-level device in the hierarchy (for example, a host controller) is limited only by the capabilities of the expander.

With a rich and functional infrastructure, SAS allows you to create complex storage topologies rather than dedicated hard drives or separate network attached storage. In this case, by “complex” we should not mean that such a topology is difficult to work with. SAS configurations consist of simple disk snap-ins or use expanders. Any SAS link can be expanded or reduced depending on bandwidth requirements. You can use both powerful SAS hard drives and capacious SATA models. Together with powerful RAID controllers, data arrays can be easily configured, expanded or reconfigured - both from a RAID level and hardware perspective.

All of this becomes even more important when you consider how quickly enterprise storage is growing. Today everyone is hearing about SAN - storage area network. It involves a decentralized organization of a data storage subsystem with traditional servers, using physically remote storage. Over existing Gigabit Ethernet or Fiber Channel networks, a slightly modified SCSI protocol is launched, encapsulated in Ethernet packets (iSCSI - Internet SCSI). The system, which runs from a single hard drive to complex nested RAID arrays, becomes a so-called target and is associated with an initiator (host system), which treats the target as if it were just a physical element.

iSCSI, of course, allows you to create a strategy for developing storage, organizing data, or managing access to it. We gain another level of flexibility by removing storage directly attached to servers, allowing any storage subsystem to become an iSCSI target. The transition to off-site storage makes the system work independent of data storage servers (a dangerous point of failure) and improves the manageability of the hardware. From a software point of view, the storage still remains "inside" the server. The iSCSI target and initiator can be located nearby, on different floors, in different rooms or buildings - it all depends on the quality and speed of the IP connection between them. From this perspective, it is important to note that a SAN is not well suited to the requirements of online applications such as databases.

2.5" SAS hard drives

2.5" hard drives for the professional sector are still perceived as a novelty. We've been looking at the first such drive from Seagate for quite some time now - 2.5" Ultra320 Savvio, which left a good impression. All 2.5" SCSI drives use a spindle speed of 10,000 rpm, but they do not reach the performance level of 3.5" drives with the same spindle speed. The fact is that the external tracks of 3.5" models rotate at a higher linear speed, which provides higher data transfer rates.

The advantage of small hard drives does not lie in capacity: today the maximum for them is still 73 GB, while with 3.5" enterprise-class hard drives we already get 300 GB. In many areas, the ratio of performance to occupied physical volume is very important or energy efficiency. The more hard drives you use, the more performance you will reap - paired with the appropriate infrastructure, of course. At the same time, 2.5" hard drives consume almost half as much energy as 3.5" competitors. If we consider the ratio performance per watt (the number of I/O operations per watt), then the 2.5" form factor gives very good results.

If you primarily need capacity, then 3.5" drives at 10,000 rpm are unlikely to be the best choice. The fact is that 3.5" SATA hard drives provide 66% more capacity (500 instead of 300 GB per hard drive), leaving performance levels acceptable. Many hard drive manufacturers offer SATA models for 24/7 operation, and the price of drives is reduced to a minimum. Reliability problems can be solved by purchasing spare drives for immediate replacement in the array.

The MAY line represents the current generation of 2.5" Fujitsu drives for the professional sector. The rotation speed is 10,025 rpm and the capacities are 36.7 and 73.5 GB. All drives come with 8 MB of cache and give an average read seek time 4.0 ms and recording 4.5 ms. As we have already mentioned, a nice feature of 2.5" hard drives is reduced power consumption. Typically, one 2.5" hard drive can save at least 60% energy compared to a 3.5" drive.

3.5" SAS hard drives

MAX is Fujitsu's current line of high-performance 15,000 rpm hard drives. So the name is quite appropriate. Unlike 2.5" drives, here we get as much as 16 MB of cache and a short average seek time of 3.3 ms for read and 3.8 ms for write. Fujitsu offers models with 36.7 GB, 73.4 GB and 146 GB (with one, two and four platters).

Hydrodynamic bearings have also made their way to enterprise-class hard drives, so the new models are significantly quieter than the previous ones at 15,000 rpm. Of course, such hard drives must be properly cooled, and the equipment also ensures this.

Hitachi Global Storage Technologies also offers its own line of high-performance solutions. The UltraStar 15K147 runs at 15,000 rpm and has a 16 MB cache, like the Fujitsu drives, but the platter configuration is different. The 36.7 GB model uses two platters rather than one, and the 73.4 GB model uses three platters rather than two. This indicates lower data density, but this design essentially eliminates the use of the inner, slowest areas of the platters. As a result, the heads have to move less, which gives better average access time.

Hitachi also offers 36.7 GB, 73.4 GB and 147 GB models with a claimed seek (read) time of 3.7 ms.

Although Maxtor has already become part of Seagate, the company's product lines are still preserved. The manufacturer offers models of 36, 73 and 147 GB, all of which feature a spindle speed of 15,000 rpm and a 16 MB cache. The company claims an average seek time of 3.4ms for read and 3.8ms for write.

Cheetah has long been associated with high-performance hard drives. Seagate was able to instill a similar association with the release of Barracuda in the desktop PC segment, offering the first desktop drive at 7200 rpm in 2000.

Available in 36.7 GB, 73.4 GB and 146.8 GB models. All of them are distinguished by a spindle speed of 15,000 rpm and an 8 MB cache. The stated average seek time for reading is 3.5 ms and for writing 4.0 ms.

Host adapters

Unlike SATA controllers, SAS components can only be found on server-class motherboards or as expansion cards for PCI-X or PCI Express. If we take one step further and consider RAID controllers (Redundant Array of Inexpensive Drives), due to their complexity they are sold, for the most part, in the form of separate cards. RAID cards contain not only the controller itself, but also a chip for accelerating calculations of redundancy information (XOR engine), as well as cache memory. Sometimes a small amount of memory is soldered onto the card (most often 128 MB), but some cards allow you to expand the capacity using DIMM or SO-DIMM.

When choosing a host adapter or RAID controller, you should clearly determine what you need. The range of new devices is growing before our eyes. Simple multiport host adapters will cost relatively little, but powerful RAID cards will cost you a lot of money. Consider where you will place the drives: External storage requires at least one external slot. Rack servers typically require low profile cards.

If you need RAID, then decide whether you will use hardware acceleration. Some RAID cards consume CPU resources to perform XOR calculations for RAID 5 or 6 arrays; others use their own hardware XOR engine. RAID acceleration is recommended for environments where the server does more than just store data, such as databases or web servers.

All host adapter cards that we presented in our article support speeds of 300 MB/s per SAS port and allow for very flexible implementation of the data storage infrastructure. External ports today will surprise few people, and take into account the support for both SAS and SATA hard drives. All three cards use the PCI-X interface, but PCI Express versions are already in development.

In our article, we paid attention to cards with eight ports, but the number of connected hard drives is not limited to this. Using a SAS expander (external) you can connect any storage. As long as a four-lane connection is sufficient, you can increase the number of hard drives up to 122. Due to the performance overhead of calculating RAID 5 or RAID 6 parity information, typical external RAID storage will not be able to sufficiently load the bandwidth of a four-lane connection, even if you use a large number of drives.

48300 is a SAS host adapter designed for the PCI-X bus. PCI-X continues to dominate the server market today, although more and more motherboards are equipped with PCI Express interfaces.

The Adaptec SAS 48300 uses the PCI-X interface at 133 MHz, which gives a throughput of 1.06 GB/s. Fast enough if the PCI-X bus is not loaded with other devices. If you include a slower device on the bus, then all other PCI-X cards will reduce their speed to the same. For this purpose, several PCI-X controllers are sometimes installed on the board.

Adaptec is positioning the SAS 4800 for mid- and low-price servers, as well as workstations. The suggested retail price is $360, which is quite reasonable. Adaptec HostRAID is supported, allowing you to migrate to the simplest RAID arrays. In this case, these are RAID levels 0, 1 and 10. The card supports an external four-channel SFF8470 connection, as well as an internal SFF8484 connector paired with a cable for four SAS devices, that is, we get eight ports in total.

The card fits into a 2U rackmount server if you install a low-profile slot blank. The package also includes a CD with a driver, a quick installation guide, and an internal SAS cable through which up to four system drives can be connected to the card.

SAS player LSI Logic sent us the SAS3442X PCI-X host adapter, a direct competitor to the Adaptec SAS 48300. It comes with eight SAS ports, which are split between two quad-channel interfaces. The “heart” of the card is the LSI SAS1068 chip. One of the interfaces is intended for internal devices, the second - for external DAS (Direct Attached Storage). The board uses the PCI-X 133 bus interface.

As usual, a 300 MB/s interface is supported for SATA and SAS drives. There are 16 LEDs on the controller board. Eight of them are simple activity LEDs, and another eight are designed to indicate system malfunction.

The LSI SAS3442X is a low-profile card, so it fits easily into any 2U rackmount server.

We note driver support for Linux, Netware 5.1 and 6, Windows 2000 and Server 2003 (x64), Windows XP (x64) and Solaris up to 2.10. Unlike Adaptec, LSI chose not to add support for any RAID modes.

RAID adapters

SAS RAID4800SAS is Adaptec's solution for more complex SAS environments and can be used for application servers, streaming servers, and more. Before us, again, is a card with eight ports, with one external four-channel SAS connection and two internal four-channel interfaces. But if an external connection is used, then only one four-channel interface remains from the internal ones.

The card is also designed for the PCI-X 133 bus, which provides sufficient bandwidth for even the most demanding RAID configurations.

As for RAID modes, here the SAS RAID 4800 easily outperforms its “little brother”: by default, RAID levels 0, 1, 10, 5, 50 are supported, if you have a sufficient number of drives. Unlike the 48300, Adaptec included two SAS cables, so you can immediately connect eight hard drives to the controller. Unlike the 48300, the card requires a full-size PCI-X slot.

If you decide to upgrade your card to Adaptec Advanced Data Protection Suite, you will have the opportunity to switch to RAID modes with double redundancy (6, 60), as well as a number of enterprise-class features: striped mirror drive (RAID 1E), hot spacing (RAID 5EE) and copyback hot spare. The Adaptec Storage Manager utility has a browser-like interface and can be used to manage all Adaptec adapters.

Adaptec offers drivers for Windows Server 2003 (and x64), Windows 2000 Server, Windows XP (x64), Novell Netware, Red Hat Enterprise Linux 3 and 4, SuSe Linux Enterprise Server 8 and 9, and FreeBSD.

SAS snap-ins

The 335SAS is a snap-in for four SAS or SATA drives, but must be connected to a SAS controller. Thanks to the 120mm fan, the drives will be well cooled. You will also have to connect two Molex power plugs to the equipment.

Adaptec has included an I2C cable that can be used to control the hardware through an appropriate controller. But with SAS drives this will no longer work. An additional LED cable is designed to signal drive activity, but, again, only for SATA drives. The package also includes an internal SAS cable for four drives, so an external four-channel cable will be sufficient to connect the drives. If you want to use SATA drives, you will have to use adapters from SAS to SATA.

The retail price of $369 cannot be called low. But you will get a solid and reliable solution.

SAS storage

SANbloc S50 is an enterprise-level solution for 12 drives. You will receive a 2U rack-mount enclosure that connects to SAS controllers. This is one of the best examples of scalable SAS solutions. 12 drives can be either SAS or SATA. Or represent a mixture of both types. The built-in expander can use one or two four-channel SAS interfaces to connect the S50 to a host adapter or RAID controller. Since this is clearly a professional solution, it is equipped with two power supplies (redundant).

If you have already purchased an Adaptec SAS host adapter, you can easily connect it to the S50 and use Adaptec Storage Manager to manage the drives. If we install 500 GB SATA hard drives, we get 6 TB of storage. If we take 300 GB SAS drives, then the capacity will be 3.6 TB. Since the expander is connected to the host controller by two four-channel interfaces, we will get a throughput of 2.4 GB/s, which will be more than enough for any type of array. If you install 12 drives in a RAID0 array, the maximum throughput will be only 1.1 GB/s. In the middle of this year, Adaptec promises to release a slightly modified version with two independent SAS I/O units.

SANbloc S50 contains automatic monitoring and automatic fan speed control. Yes, the device is a bit loud, so we were relieved to get it back from the lab after testing was completed. The drive failure message is sent to the controller via SES-2 (SCSI Enclosure Services) or via the I2C physical interface.

Operating temperatures for drives are 5-55°C, and for accessories - from 0 to 40°C.

At the beginning of our tests, we got a peak throughput of just 610 MB/s. By changing the cable between the S50 and the Adaptec host controller, we were still able to achieve 760 MB/s. To load the system in RAID 0 mode, we used seven hard drives. Increasing the number of hard drives did not lead to an increase in throughput.

Test configuration

System hardware
Processors	2x Intel Xeon (Nocona core) 3.6 GHz, FSB800, 1 MB L2 cache
Platform	Asus NCL-DS (Socket 604) Chipset Intel E7520, BIOS 1005
Memory	Corsair CM72DD512AR-400 (DDR2-400 ECC, reg.) 2x 512 MB, CL3-3-3-10
System hard drive	Western Digital Caviar WD1200JB 120 GB, 7200 rpm, 8 MB cache, UltraATA/100
Drive controllers	Intel 82801EB UltraATA/100 (ICH5) controller Promise SATA 300TX4 Driver 1.0.0.33 Adaptec AIC-7902B Ultra320 Driver 3.0 Adaptec 48300 8 port PCI-X SAS Driver 1.1.5472 Adaptec 4800 8 port PCI-X SAS Driver 5.1.0.8360 Firmware 5.1.0.8375 LSI Logic SAS3442X 8 port PCI-X SAS Driver 1.21.05 BIOS 6.01
Storage	4-bay hot-swappable indoor hardware 2U, 12-HDD SAS/SATA JBOD
Net	Broadcom BCM5721 Gigabit Ethernet
Video card	Built-in ATi RageXL, 8 MB
Tests
performance measurement	c"t h2benchw 3.6
Measuring I/O Performance	IOMeter 2003.05.10 Fileserver-Benchmark Webserver-Benchmark Database-Benchmark Workstation-Benchmark
System software and drivers
OS	Microsoft Windows Server 2003 Enterprise Edition, Service Pack 1
Platform Driver	Intel Chipset Installation Utility 7.0.0.1025
Graphics driver Workstation scenario. After reviewing several new SAS hard drives, three associated controllers, and two snap-ins, it became clear that SAS is truly a promising technology. If you refer to the SAS technical documentation, you will understand why. This is not only a successor to SCSI with a serial interface (fast, convenient and easy to use), but also an excellent level of scaling and infrastructure expansion, in comparison with which Ultra320 SCSI solutions seem like the stone age. And the compatibility is simply excellent. If you are planning to purchase professional SATA equipment for your server, then you should take a closer look at SAS. Any SAS controller or hardware is compatible with both SAS and SATA hard drives. Therefore, you can create either a high-performance SAS environment or a high-capacity SATA environment - or both at once. Convenient support for external storage is another important advantage of SAS. If SATA storages use either some proprietary solutions or a single SATA/eSATA channel, the SAS storage interface allows you to increase the connection throughput in groups of four SAS channels. As a result, we get the opportunity to increase bandwidth to suit the needs of applications, and not be limited by 320 MB/s UltraSCSI or 300 MB/s SATA. Moreover, SAS expanders allow you to create an entire hierarchy of SAS devices, so administrators have greater freedom of action. The evolution of SAS devices will not end there. It seems to us that the UltraSCSI interface can be considered outdated and slowly written off. It is unlikely that the industry will improve it, unless it continues to support existing UltraSCSI implementations. Still, new hard drives, the latest models of storage and equipment, as well as an increase in interface speed to 600 MB/s, and then up to 1200 MB/s - all this is intended for SAS. What should a modern storage infrastructure look like? With the availability of SAS, UltraSCSI's days are numbered. The sequential version is a logical step forward and copes with all tasks better than its predecessor. The question of choosing between UltraSCSI and SAS becomes obvious. Choosing between SAS or SATA is somewhat more difficult. But if you look into the future, then SAS components will still be better. Indeed, for maximum performance or from the point of view of scalability prospects, there is no alternative to SAS today.

Why SAS?

The Serial Attached SCSI interface is not just a serial implementation of the SCSI protocol. It does much more than simply port SCSI features such as Tagged Command Queuing (TCQ) through a new connector. If we wanted the greatest simplicity, then we would use the Serial ATA (SATA) interface, which is a simple point-to-point connection between the host and an end device such as a hard drive.

But SAS is based on an object model that defines a "SAS domain" - a data delivery system that can include optional expanders and SAS end devices such as hard drives and host bus adapters (HBA). from SATA, SAS devices can have multiple ports, each of which can use multiple physical connections to provide faster (wider) SAS connections.Moreover, any given target can be accessed by multiple initiators, and cable lengths can be up to eight meters ( for the first generation of SAS) versus one meter for SATA. It is clear that this provides many opportunities for creating high-performance or redundant storage solutions. In addition, SAS supports the SATA Tunneling Protocol (STP), which allows you to connect SATA devices to the SAS controller.

The second generation SAS standard increases connection speeds from 3 to 6 Gbps. This speed increase is very important for complex environments where high performance is required due to high-speed storage. The new version of SAS also aims to reduce cabling complexity as well as the number of connections per Gbps of bandwidth by increasing possible cable lengths and improving expander performance (zoning and auto-discovery). We'll talk about these changes in detail below.

Increase SAS speed up to 6 Gbps

To bring the benefits of SAS to a wider audience, the SCSI Trade Association (SCSI TA) presented a primer on SAS technology at the Storage Networking World Conference earlier this year in Orlando, Florida, USA. The so-called SAS Plugfest, where 6 Gbps SAS operation, compatibility and functions were demonstrated, took place even earlier in November 2008. LSI and Seagate were the first on the market to introduce hardware compatible with 6 Gbps SAS, but other manufacturers should catch up soon. In our article we will look at the current state of SAS technologies and some new devices.

SAS Features and Basics

SAS Fundamentals

Unlike SATA, the SAS interface operates on a full duplex basis, providing full bandwidth in both directions. As mentioned earlier, SAS connections are always established through physical connections using unique device addresses. In contrast, SATA can only address port numbers.

Each SAS address can contain multiple physical layer (PHY) interfaces, allowing for broader connections via InfiniBand (SFF-8470) or mini-SAS cables (SFF-8087 and -8088). Typically, four SAS interfaces with one PHY each are combined into one wide SAS interface, which is already connected to the SAS device. Communication can also be done through expanders, which act more like switches than SAS devices.

Features such as zoning now allow administrators to associate specific SAS devices with initiators. This is where the increased throughput of 6Gbps SAS will come in handy, as a quad-link connection will now have twice the speed. Finally, SAS devices can even have multiple SAS addresses. Since SAS drives can use two ports, with one PHY on each, the drive can have two SAS addresses.

Connections and Interfaces

Click on the picture to enlarge.

Addressing of SAS connections occurs through SAS ports using SSP (Serial SCSI Protocol), but communication at the lower level from PHY to PHY is done using one or more physical connections for reasons of increased bandwidth. SAS uses 8/10-bit encoding to convert 8 bits of data into 10-character transmissions for the purposes of timing recovery, DC balance, and error detection. As a result, we get an effective throughput of 300 MB/s for 3 Gb/s transfer mode and 600 MB/s for 6 Gb/s connections. Fiber Channel, Gigabit Ethernet, FireWire and other technologies operate using a similar encoding scheme.

The power and data interfaces of SAS and SATA are very similar to each other. But if SAS has data and power interfaces combined into one physical interface (SFF-8482 on the device side), then SATA requires two separate cables. The gap between the power and data pins (see illustration above) in the case of SAS is closed, which does not allow connecting a SAS device to a SATA controller.

On the other hand, SATA devices can work fine on a SAS infrastructure thanks to STP or in native mode if expanders are not used. STP adds additional latency to the expanders because they need to establish a connection, which is slower than a direct SATA connection. However, the delays are still very small.

Domains, expanders

SAS domains can be thought of as tree structures, much like complex Ethernet networks. SAS expanders can handle a large number of SAS devices, but they use circuit switching rather than the more common packet switching. Some expanders contain SAS devices, others do not.

SAS 1.1 recognizes edge expanders, which allow a SAS initiator to bind to up to 128 additional SAS addresses. In a SAS 1.1 domain, you can only use two edge expanders. However, one fanout expander can connect up to 128 edge expanders, which significantly increases the infrastructure capabilities of your SAS solution.

Click on the picture to enlarge.

Compared to SATA, the SAS interface may seem complex: different initiators access target devices through expanders, which implies laying out appropriate routes. SAS 2.0 simplifies and improves routing.

Remember that SAS does not allow loops or multiple paths. All connections must be point-to-point and exclusive, but the connection architecture itself is highly scalable.

New SAS 2.0 Features: Expanders, Performance

	SAS 1.0/1.1
Function	Retains legacy SCSI support SATA compatible	Compatible with 3 Gbps Improved speed and signal transmission Zone management Improved scalability
Storage functions	RAID 6 Small form factor HPC High Capacity SAS Drives Ultra320 SCSI replacement Choice: SATA or SAS Blade servers	RAS (data security) Safety (FDE) Cluster support Support for larger topologies SSD Virtualization External storage 4K sector size
Data transfer speed and cable bandwidth	4 x 3 Gbit/s (1.2 GB/s)	4 x 6 Gbit/s (2.4 GB/s)
Cable type	Copper	Copper
Length of cable	8 m	10 m

Expander zones and automatic configuration

Edge and fanout expanders are almost a thing of history. This is often attributed to updates in SAS 2.0, but the reason actually lies in SAS zones introduced in 2.0, which remove the separation between edge and extension expanders. Of course, zones are usually implemented specifically for each manufacturer, and not as a single industry standard.

In fact, now several zones can be located on one information delivery infrastructure. This means that storage targets (drives) can be accessed by different initiators through the same SAS expander. Domain segmentation is done through zones and access is exclusive.

In this article, we take a look at the future of SCSI and look at some of the advantages and disadvantages of SCSI, SAS, and SATA interfaces.

In fact, the issue is a little more complex than simply replacing SCSI with SATA and SAS. Traditional parallel SCSI is a tried and tested interface that has been in use for a long time. Currently, SCSI offers very fast data transfer rates of 320 Megabytes per second (Mbps) using the modern Ultra320 SCSI interface. In addition, SCSI offers a wide range of features, including Command-Tag Queuing (a method of optimizing I/O commands to increase performance). SCSI hard drives are reliable; over a short distance it is possible to create a daisy chain of 15 devices connected to a SCSI link. These features make SCSI an excellent choice for productivity desktops and workstations, all the way up to enterprise servers today.

SAS hard drives use the SCSI command set and have the same reliability and performance as SCSI drives, but use a serial version of the SCSI interface, with a speed of 300 MB/sec. Although it is slightly slower than SCSI at 320 Mbps, the SAS interface is capable of supporting up to 128 devices over longer distances than Ultra320 and can expand to 16,000 devices per channel. SAS hard drives offer the same reliability and rotation speeds (10000-15000) as SCSI drives.

SATA drives are a little different. Where SCSI and SAS drives focus on performance and reliability, SATA drives sacrifice these in favor of significantly increasing capacity and reducing cost. For example, the SATA drive has now reached a capacity of 1 terabyte (TB). SATA is used where maximum capacity is needed, such as for data backup or archiving. SATA now offers point-to-point connections at speeds of up to 300 Mbps, and easily outpaces the traditional ATA parallel interface's 150 Mbps.

So what will happen to SCSI? It works great. The problem with traditional SCSI is that it is simply reaching the end of its life. Parallel SCSI, which has a speed of 320 MB/sec, will not be significantly faster on current SCSI cable lengths. For comparison, SATA drives will reach speeds of 600 MB/sec in the near future, SAS has plans to reach 1200 MB/sec. SATA drives can also work with a SAS interface, so these drives can be used simultaneously in some storage systems. The potential for increased expandability and data transfer performance far exceeds that of SCSI. But SCSI isn't going away anytime soon. We'll continue to see SCSI in small and midsize servers for several years to come. As hardware is upgraded, SCSI will be systematically replaced by SAS/SATA drives for faster speeds and easier connections.