This article aims to explain the difference between the types of hard drives and help you make the right choice when buying a dedicated server.

SATA - Serial ATA

Currently, SATA drives are used on most personal computers in the world and on budget server hardware configurations. Compared to SAS and SSD disks, the read and write speed of SATA disks is noticeably lower, but they are chosen because of the large amounts of stored information.

SATA drives are well suited for game servers that do not require frequent writing and reading of information. It is also advisable to use SATA drives for the following purposes:

streaming operations such as video encoding;
data storage;
backup systems;
voluminous, but not loaded file servers.

SAS - Serial Attached SCSI

SAS drives are designed from the ground up for enterprise and industrial workloads, which positively impacts their performance. SAS drives rotate at twice the speed of SATA drives, so they should be chosen for tasks that are speed sensitive and require multi-threaded access. Also, SAS drives (unlike SSDs) can provide reliable and repeatable data rewriting.

SAS drives are optimal for hosting because they can provide high reliability of data storage. In addition, SAS hard drives are well suited for the following tasks:

database management systems (DBMS);
High load WEB servers;
distributed systems;
systems that process a large number of requests - terminal servers, 1C servers.

The only drawback of SAS drives (like SSDs) is their small size and high price.

SSD - Solid-state Drive

Recently, SSDs are becoming more and more popular. SSD does not use magnetic disks for recording, but contains only non-volatile memory chips, similar to those used in USB flash drives.

SSDs have no moving parts, which provides high mechanical durability, reduced power consumption and high speed. At the moment, SSD drives provide the highest possible read and write speed, which allows them to be used for any high-load projects.

The main disadvantage of SSD drives is that they are limited in the amount of information that can be rewritten to the drive. Accordingly, if your system overwrites more than 20 GB of data per day, be prepared to replace the SSD after a while. By the way, the price of such discs is higher than that of both of the above types.

When generating a page, many modern CMS often require simultaneous access to several files on disk. It is for such systems that SSD drives are the ideal choice. Using SSD drives for busy sites is a guarantee that you get the maximum read speed.

This article will focus on what allows you to connect a hard drive to your computer, namely, the hard drive interface. More precisely, about the interfaces of hard drives, because a great variety of technologies for connecting these devices have been invented over the entire period of their existence, and the abundance of standards in this area can confuse an inexperienced user. However, about everything in order.

Hard disk interfaces (or, strictly speaking, external storage interfaces, since they can be used not only, but also other types of drives, for example, optical drives) are designed to exchange information between these external memory devices and the motherboard. Hard drive interfaces, as much as the physical parameters of the drives, affect many of the drive's performance and performance. In particular, the interfaces of the drives determine their parameters such as the speed of data exchange between the hard drive and the motherboard, the number of devices that can be connected to a computer, the ability to create disk arrays, the possibility of hot plugging, support for NCQ and AHCI technologies, etc. ... It also depends on the hard disk interface which cable, cord or adapter you need to connect it to the motherboard.

SCSI - Small Computer System Interface

SCSI is one of the oldest interfaces developed for connecting storage devices in personal computers. This standard appeared in the early 1980s. One of its developers was Alan Shugart, also known as the inventor of floppy drives.

External view of the SCSI interface on the board and the cable for connecting to it

The SCSI standard (traditionally this abbreviation is read in Russian transcription as "fairy tale") was originally intended for use in personal computers, as evidenced by the very name of the format - Small Computer System Interface, or system interface for small computers. However, it so happened that drives of this type were used mainly in top-class personal computers, and later in servers. This was due to the fact that, despite the successful architecture and a wide set of commands, the technical implementation of the interface was rather complicated and did not suit the cost of mass PCs.

However, this standard had a number of capabilities that are not available for other types of interfaces. For example, a Small Computer System Interface cable can have a maximum length of 12 m and a data transfer rate of 640 MB / s.

Like the later IDE interface, the SCSI interface is parallel. This means that the interface uses buses that carry information over multiple conductors. This feature was one of the limiting factors for the development of the standard, and therefore, as its replacement, the more advanced, serial SAS (from Serial Attached SCSI) standard was developed.

SAS - Serial Attached SCSI

This is how the SAS interface of a server disk looks like

Serial Attached SCSI was developed as an improvement on the rather old Small Computers System Interface for connecting hard drives. Despite the fact that Serial Attached SCSI takes advantage of the main advantages of its predecessor, it nevertheless has many advantages. Among them are the following:

Use of a common bus by all devices.
The serial communication protocol used by SAS allows fewer signal lines to be used.
No bus termination required.
Almost unlimited number of connected devices.
Higher bandwidth (up to 12 Gbps). Future implementations of SAS are expected to support data rates up to 24 Gbps.
The ability to connect to the SAS controller drives with Serial ATA interface.

Typically, Serial Attached SCSI systems are built around several components. The main components include:

Target devices. This category includes the actual drives or disk arrays.
Initiators are microcircuits designed to generate requests to target devices.
Data delivery system - cables connecting target devices and initiators

Serial Attached SCSI connectors come in different shapes and sizes, depending on the type (external or internal) and the SAS versions. Below is the internal SFF-8482 connector and the external SFF-8644 connector designed for SAS-3:

Left - internal SAS SFF-8482 connector; On the right is an external SAS SFF-8644 connector with a cable.

Some examples of the appearance of SAS cords and adapters: HD-Mini SAS cord and SAS-Serial ATA adapter cord.

Left - HD Mini SAS cord; Right - SAS to Serial ATA adapter cable

Firewire - IEEE 1394

Today it is quite common to find hard drives with a Firewire interface. Although you can connect any type of peripheral device to your computer via the Firewire interface, and it cannot be called a specialized interface designed for connecting exclusively hard drives, nevertheless, Firewire has a number of features that make it extremely convenient for this purpose.

FireWire - IEEE 1394 - Laptop View

The Firewire interface was developed in the mid-1990s. The beginning of the development was laid by the well-known company Apple, which needed its own, other than USB, bus for connecting peripheral equipment, primarily multimedia. The specification that describes the operation of the Firewire bus is called IEEE 1394.

Firewire is one of the most commonly used high-speed serial external bus formats today. The main features of the standard include:

Hot-pluggable devices.
Open bus architecture.
Flexible topology for connecting devices.
Data transfer rates varying over a wide range - from 100 to 3200 Mbps.
The ability to transfer data between devices without a computer.
The possibility of organizing local networks using a bus.
Bus power transmission.
A large number of connected devices (up to 63).

To connect hard drives (usually through external hard drive enclosures) via the Firewire bus, as a rule, a special SBP-2 standard is used, which uses the Small Computers System Interface protocol command set. It is possible to connect Firewire devices to a regular USB connector, but this requires a special adapter.

IDE - Integrated Drive Electronics

The abbreviation IDE is undoubtedly familiar to most personal computer users. The interface standard for connecting IDE hard drives was developed by the well-known hard drive manufacturer Western Digital. The advantage of IDE over other interfaces that existed at that time, in particular, the Small Computers System Interface, as well as the ST-506 standard, was that there was no need to install a hard disk controller on the motherboard. The IDE standard meant installing a drive controller on the drive's case, and the motherboard had only a host interface adapter for connecting IDE drives.

IDE interface on the motherboard

This innovation has improved the performance of the IDE drive due to the fact that the distance between the controller and the drive itself is reduced. In addition, the installation of an IDE controller inside the hard drive case made it possible to somewhat simplify both motherboards and the production of the hard drives themselves, since the technology gave manufacturers freedom in terms of the optimal organization of the drive logic.

The new technology was originally called Integrated Drive Electronics. Subsequently, a standard describing it was developed, called ATA. This name comes from the last part of the name of the PC / AT family of computers by adding the word Attachment.

A dedicated IDE cable is used to connect a hard drive or other device, such as an optical drive that supports Integrated Drive Electronics, to the motherboard. Since ATA refers to parallel interfaces (therefore it is also called Parallel ATA or PATA), that is, interfaces providing for the simultaneous transfer of data over several lines, its data cable has a large number of conductors (usually 40, and in the latest versions of the protocol it was possible to use 80-wire cable). The typical data cable for this standard is flat and wide, but there are also round cables. The power cable for Parallel ATA drives has a 4-pin connector and is connected to the computer's power supply.

The following are examples of IDE cable and round PATA data cable:

External view of the interface cable: on the left - flat, on the right in a round braid - PATA or IDE.

Due to the comparative cheapness of Parallel ATA drives, the ease of implementation of the interface on the motherboard, as well as the ease of installation and configuration of PATA devices for the user, Integrated Drive Electronics drives for a long time ousted devices of other types of interface from the market of hard drives for budget personal computers.

However, the PATA standard also has several disadvantages. First of all, this is a limitation on the length that a Parallel ATA data cable can have - no more than 0.5 m. In addition, the parallel organization of the interface imposes a number of restrictions on the maximum data transfer rate. Does not support the PATA standard and many of the advanced features that other interface types have, such as hot-plugging devices.

SATA - Serial ATA

SATA interface on the motherboard

The SATA (Serial ATA) interface, as you might guess from the name, is an improvement over ATA. This improvement consists, first of all, in converting the traditional parallel ATA (Parallel ATA) into a serial interface. However, the differences between the Serial ATA standard and the traditional one are not limited to this. In addition to changing the data transfer type from parallel to serial, the data and power connectors have also changed.

Below is the SATA data cable:

Data cable for SATA interface

This made it possible to use a significantly longer cable and increase the data transfer rate. However, the downside was the fact that PATA devices, which were present on the market in huge quantities before the advent of SATA, became impossible to directly plug into the new connectors. True, most new motherboards still have old connectors and support connecting older devices. However, the reverse operation - connecting a new type of drive to an old motherboard usually causes much more problems. For this operation, the user usually needs a Serial ATA to PATA adapter. The power cable adapter is usually relatively simple in design.

Serial ATA to PATA Power Adapter:

On the left is a general view of the cable; On the right, the appearance of the PATA and Serial ATA connectors is enlarged

More complex, however, is the case with a device such as an adapter for connecting a serial device to a parallel interface connector. Typically, this type of adapter is made in the form of a small microcircuit.

External view of the universal bidirectional adapter between SATA - IDE interfaces

Nowadays the Serial ATA interface has practically supplanted Parallel ATA, and PATA drives can now be found mainly only in fairly old computers. Another feature of the new standard that made it so popular was support.

Type of adapter from IDE to SATA

You can tell a little more about NCQ technology. The main advantage of NCQ is that it allows you to use ideas that have long been implemented in the SCSI protocol. In particular, NCQ supports a system for sequencing read / write operations to multiple drives installed in the system. Thus, NCQ can significantly improve the performance of storage devices, especially hard disk arrays.

SATA to IDE adapter

To use NCQ, technology support is required from the hard drive side as well as the motherboard host adapter. Almost all adapters that support AHCI also support NCQ. In addition, some older proprietary adapters support NCQ. Also, for the operation of NCQ, its support from the operating system is required.

eSATA - External SATA

Separately, it is worth mentioning the eSATA (External SATA) format, which seemed promising at one time, but did not receive wide distribution. As you might guess from the name, eSATA is a type of Serial ATA designed for connecting exclusively external drives. The eSATA standard offers for external devices most of the capabilities of the standard, i.e. internal Serial ATA, in particular, the same signal and command system and the same high speed.

ESATA connector on laptop

However, eSATA has some differences from the internal bus standard that gave rise to it. In particular, eSATA supports longer data cables (up to 2m) and also has higher power requirements for drives. In addition, eSATA connectors are slightly different from standard Serial ATA connectors.

Compared to other external buses such as USB and Firewire, eSATA, however, has one major drawback. Whereas these buses allow the device to be powered through the bus cable itself, the eSATA drive requires dedicated power connectors. Therefore, despite the relatively high data transfer rate, eSATA is currently not very popular as an interface for connecting external drives.

Conclusion

Information stored on a hard disk cannot become useful to the user and available to application programs until it is accessed by the computer's central processor. Hard drive interfaces are the means for communication between these drives and the motherboard. Today, there are many different types of hard drive interfaces, each of which has its own advantages, disadvantages and characteristic features. We hope that the information given in this article will be in many ways useful to the reader, because the choice of a modern hard disk is largely determined not only by its internal characteristics, such as capacity, cache memory, access and rotation speed, but also by the interface for which it was designed.

For over 20 years, the parallel bus interface has been the most common communication protocol for most digital storage systems. But as the demand for bandwidth and flexibility has grown, the shortcomings of the two most common parallel technologies: SCSI and ATA have become apparent. The lack of compatibility between the parallel SCSI and ATA interfaces — different connectors, cables, and instruction sets used — increases the cost of maintaining systems, research and development, training, and qualifying new products.

Today, parallel technologies still satisfy users of modern enterprise systems in terms of performance, but the growing needs for higher speeds, better data security during transmission, reduced physical size, as well as wider standardization, call into question the ability of a parallel interface without unnecessary costs to keep up with rapidly growing CPU performance and the speed of hard drives. In addition, with austerity, it is becoming increasingly difficult for businesses to fundraise and maintain heterogeneous rear panel connectors for server chassis and external disk enclosures, validate heterogeneous interfaces for compatibility, and inventory dissimilar connections for I / O operations.

The use of parallel interfaces also presents a number of other problems. Parallel data transmission over a wide stub cable is prone to crosstalk, which can create additional interference and lead to signal errors - to avoid this trap, you have to slow down the signal speed or limit the cable length, or 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 accumulator in order to prevent signal reflection 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 inherent in traditional parallel interfaces. These new technologies got their name from the method of signal transmission, when all information is transmitted sequentially (English serial), a single stream, in contrast to multiple streams, which are used in parallel technologies. The main advantage of the serial interface is that when data is transferred in a single stream, it moves much faster than using the parallel interface.

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

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

SAS, the successor to parallel SCSI, builds on the proven functionality of its predecessor and promises to greatly expand the capabilities of today's enterprise storage systems. SAS has many advantages that traditional storage solutions do not offer. In particular, SAS allows up to 16,256 devices to be connected to a single port and provides reliable point-to-point serial connections at speeds up to 3Gb / s.

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

SAS improves the addressing and connectivity of drives with hardware extenders that allow you to connect a large number of drives to one or more host controllers. Each expander can connect up to 128 physical devices, which can be other host controllers, other SAS expanders, or disk drives. This design scales well and allows you to create enterprise-scale topologies that easily support multisite clustering for automatic system recovery in the event of a failure and for load balancing.

One of the major 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 spending additional money 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 SATA compatible in form factor. This allows both SAS and SATA drives to be directly connected to the SAS system and thus use the system for either 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, which provides 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.

In addition, the similar physical parameters of the SAS and SATA interfaces allow for a new universal SAS rear panel that allows for both SAS and SATA drives. As a result, there is no need to use two different rear panels for SCSI and ATA drives. This interoperability benefits both backplane manufacturers and end-users by reducing hardware and design 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 which device is being accessed. The first is the Serial SCSI Protocol SSP, which sends SCSI commands, and the second is the SCSI Management Protocol SMP, which transfers control information to the expanders. The third, SATA Tunneled Protocol STP, establishes a connection that allows the transmission of SATA commands. Using these three protocols, the SAS interface is fully compatible with existing SCSI applications, management software and SATA devices.

This multi-protocol architecture, coupled with the physical compatibility of SAS and SATA connectors, makes SAS technology a versatile glue between SAS and SATA devices.

Benefits of compatibility

SAS and SATA interoperability offers a number of benefits to system designers, assemblers and end users.

System designers can use the same rear panels, connectors and cable connections due to SAS and SATA compatibility. Upgrading a system from SATA to SAS is essentially a matter of replacing disk drives. In contrast, for traditional parallel users, moving from ATA to SCSI means replacing back panels, connectors, cables, and drives. Other cost-effective benefits of sequential technology interoperability include simplified certification and material 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. What's more, the added flexibility to choose the best price / performance ratio will enable 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 right price / performance ratio. SATA drives will be the best choice for low-cost servers and storage systems, while SAS drives will provide maximum performance, reliability, and management software compatibility. The ability to upgrade from SATA drives to SAS drives without the need to purchase a new system greatly simplifies the purchasing decision process, protects your system investment and lowers your total cost of ownership.

Co-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 two organizations are working together, as well as the joint efforts of storage vendors and standards committees, to provide even more precise interoperability guidelines to help system designers, IT professionals, and end users fine-tune their systems to achieve 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 are expected to hit the market in the first half of 2004.

What is SAS, the Background It's time to acknowledge the obvious fact that the SCSI standard, even in the most modern implementations like Ultra320 SCSI, has exhausted its capabilities. At the very least, further scaling its performance, if theoretically possible, will be very expensive. The situation with this highly respected standard looks especially depressing against the background of the rapid development of all computer technology and the architecture and topology of data storage systems in particular.

Two key factors that are pushing manufacturers to improve the interfaces of hard drives are the growing performance of storage systems with a large number of transactions being processed and the speed of data retrieval from large libraries. Of course, "a holy place is never empty," and the emergence of interfaces like optical FCAL or serial SATA to some extent eliminated bottlenecks and added variety to the list of storage architectures. However, users accustomed to the capabilities of SCSI are still fans of this standard. Moreover, a lot of money has been invested in its development.

These are the preconditions for the emergence of a new industrial standard called serial-Attached SCSI, or simply SAS.

For the sake of fairness, it should be noted that the new standard did not appear suddenly and immediately: the official announcement of SAS technology, which took place on January 28, 2004, was preceded by a serious work of a development team from different companies and industrial groups - the SCSI Trade Association (STA) and the International Committee for Information Technology Standards (INCITS), sponsored by the American National Standards Institute (ANSI). First talked about the new standard in December 2001, when the board of directors of the SCSI Trade Association (STA) voted to define the Serial Attached SCSI specifications. Further, on May 2, 2002, the development of the standard was transferred to the T10 committee at INCITS (InterNational Committee for Information Technology Standards) created specifically to support, develop and promote SAS, and the first draft SAS specifications were published in mid-2003.

So, the most important thing to rely on when trying to formulate a definition of the SAS standard: Serial-Attached SCSI is a logical and natural sequential extension of the parallel SCSI interface technology used to connect peripherals to computers.
From this, to begin with, and push off.

SAS purpose

To determine the purpose of the SAS standard and its place among modern peripheral interfaces, we turn to the wording set forth in the Serial Attached SCSI FAQ on the T10 website.

Serial Attached SCSI is a logical evolution of modern interfaces and is designed for use in industrial data collection and storage centers. The SAS standard builds on the electrical and physical characteristics of the Serial ATA interface to provide scalability, performance, reliability, and data manageability in servers and storage subsystems. The architectural similarity with SATA does not prevent SAS from possessing the most demanded features of SCSI, at the same time getting rid of its disadvantages: large connectors, short length of connecting cables, limited performance and addressing.

In a broad sense, SAS is a kind of full-duplex SATA with support for two ports, high addressing capabilities, enhanced reliability, performance and logical compatibility with SCSI. Serial ATA, on the other hand, can be thought of as a simplified subset of Serial Attached SCSI for use in simple systems without critical reliability and performance requirements. This does not mean that Serial Attached SCSI devices cannot be used in conventional workstations and desktop PCs, only the presence of an appropriate host adapter is required.

In fact, Serial Attached SCSI is SCSI, but not with the usual parallel, but with a point-to-point serial architecture, with direct connection of the controller to the drives. SAS supports up to 128 drives of various types and sizes, connected together with thinner and longer (than in the case of SCSI) cables. While SCSI pushes data through its wires at a rate of about 20 MB / s, and half-duplex SATA of the first generation - 1.5 GB / s in one direction per unit of time, full-duplex SAS signaling serial interface with hot-plug support in the current implementation provides data exchange at speeds up to 3.0 Gb / s per port.

The key difference between SAS and SCSI is the ability to connect SAS drives simultaneously to two different ports, each representing a different SAS domain. You can imagine how significant this impacts on storage reliability and system resiliency. In addition, the "switch" nature of the SAS architecture allows, in theory, to connect thousands of drives "casually" (up to 16384 drives without sacrificing performance!), Which makes the scalability of such systems theoretically unlimited. The main differences between SCSI and SAS technologies are shown in the table below.

SAS Connector and Cable Specifications

One of the key features of the SAS interface during its development was the possibility of a significant increase in the data exchange rate. The next-generation SAS specifications currently under development involve data transfer rates up to 6.0 GB / s with full compatibility with the first generation of SAS devices. The next generation has not yet been seriously considered, but there is talk of the possibility of achieving data exchange rates up to 12 GB / s.

When developing connectors for SAS devices, a promising increase in the data exchange rate was laid, and at the same time, the experience of miniaturization, seen in the SATA specifications, was taken into account. The specificity of the connector lies in the placement of the second data port, since each of the ports of the SAS device is located in different domains and serves to organize independent paths from one SAS device to another to ensure trouble-free operation. In the event that one of the drives in the chain fails, this in no way affects the operation of other devices. Thus, the SAS peripheral connector design was born, in fact having an architectural similarity to the 68-pin connectors for drives with a classic parallel SCSI or SCA-2 interface, but at the same time, by analogy with SATA, which supports hot-plugging "and reliable contact.

SAS cabling is much more compact than parallel ATA and SCSI, resulting in less confusion and better airflow around the components inside the chassis. The typical length of SAS interface cables for applications such as workstations does not exceed 1 m, the maximum length of such a cable can be up to 8 m.In theory, this is comparable to the cable length for the SCSI interface, since some modern devices allow a connection between the host controller and SCSI -peripheral at a distance of more than 8 m. However, if necessary, the distance between SAS devices can be significantly increased due to the so-called SAS expanders - a kind of "pipeline pumping stations".

It is interesting to note that when developing the SAS specifications, the working group immediately took into account the need to determine the parameters of connectors and cables not only for internal, but also for external connections, similar to modern SCSI options like "server-JBOD system". For the SATA interface, the adoption of such specifications was postponed, and, as a result, the development of External SATA has not yet been completed.

As for external SAS connections, the basis was taken from Infiniband's proposal, where external connectors and cabling are designed for 4 devices and at the same time provide the performance of the first generation of external SAS connections at the level of 1.2 GB / s in each direction, that is up to 2400 MB / s in full duplex mode! Agree, more than impressive for an external interface.

SAS system topology

The use of point-to-point configurations allows obtaining high throughput, however, the reverse side of the coin is the organization of a specific topology, where the interaction of initiating (host) devices and peripherals implies support for more than two devices "in a bundle". During the development of the SAS standard, the specification immediately laid down the existence of inexpensive expanders that allow you to create systems with more than one initiating host, supporting more than one peripheral device.

Another important goal that the developers of the new standard set themselves was to get away from the limitation of the classic SCSI, which implies no more than 16 devices in one chain. As a result, each SAS system, with the appropriate number of expanders, can support addressing up to 16256 devices in a single SAS domain. It is worth noting the flexibility of the configuration of SAS expanders: their specifications imply the creation of heterogeneous systems, where both SAS and SATA devices can coexist as peripheral drives. Agree, it is very convenient, especially when building budget storage systems or devices with future-oriented scaling.

Illustration for the SAS domain organization principle
maximum capacity

Pay attention to the illustration above: the dark green module in the center represents the fanout expander. Such a "switching" expander can be present in one SAS domain in a single quantity and combine up to 128 SAS devices. However, it is not necessary to understand SAS devices exclusively as hard drives, since here we mean any possible combination of so-called "edge expanders" (light green modules), initiators and the actual drives. Peripheral expanders, in turn, can also support up to 128 SAS devices, however, no more than one additional expander can be connected to them. Initiators (hosts) are marked with blue modules on the diagram, and SAS or SATA drives are marked with brown cylinders.

SAS Protocols

The creation of a new topology and new interfaces led to the creation of a completely new definition of how to address all possible ports in a SAS domain. With parallel SCSI, of course, everything is easier, since the addressing of all devices in the domain is predefined at the hardware level.

As a result, the working group for the development of the SAS protocol decided to choose globally unique 64-bit names - WWN (WorldWide Name) for all types of SAS devices - as identifiers. Again, nothing new under the Moon, it is this addressing that has long been used when naming Fiber Channel devices.

Thus, at the moment of power-up, all devices combined into a single SAS space exchange their WWNs with each other, and only after that the set of SAS devices becomes a "meaningful" SAS system. Adding a new device to the SAS system (adding in this case means just "hot plugging") or removing it from the system results in a notification that notifies all initiators of the event and allows the system to be adjusted to the new configuration. Expanders, in turn, are responsible for "issuing" WWNs to all SATA devices in the system, both in the case of turning it on and in the case of hot plugging of a new device. Upon completion of the system initialization process, SATA devices communicate using SATA protocols, for SAS devices, the SAS protocol is used, described in other SCSI standards such as SPI (SCSI Parallel Interface).

Then everything is simpler: the exchange of commands, data, statuses and other information between SAS devices is carried out in packets, the specifications of which are very similar to the characteristics of packets for information exchange when working with parallel SCSI or Fiber Channel devices. The format of SAS data packets, called "frames", is especially similar to the Fiber Channel specifications: each of them consists of command descriptor blocks (CDBs) and other SCSI constructs defined by other SCSI standards such as the SCSI Primary Command Set or SCSI Block Command. Here's another benefit from the SAS standard: the use of a SCSI-like protocol and architecture allows you to combine SAS constructs with other storage and data processing systems with Infiniband, iSCSI or Fiber Channel architecture, which, in fact, are also SCSI objects.

The SAS protocol contains four traditional layers: the phy layer, the link layer, the port layer, and the transport layer. The aggregation of the four layers in each SAS port means that the programs and drivers used to work with the parallel SCSI ports can be used equally well to serve the SAS ports with only minor modifications.

SAS architecture

Application layers, including drivers and applications themselves, create specific tasks for the transport layer, which, in turn, encapsulates commands, data, statuses, etc. in SAS frames and delegates their transfer to the port layer. Of course, the transport layer is also responsible for receiving SAS frames from the port layer, disassembling the received frames, and delivering content to the application layer.

The SAS port layer is responsible for exchanging data packets with the link layer in the order of establishing connections, as well as for choosing the physical layer through which packets will be transmitted simultaneously to several devices. SAS physical layer means the corresponding hardware environment - transceivers and encoding modules that connect to the physical SAS interface and send signals over wired circuits.

By the way, let me remind you that at the physical level, connections in the case of the SAS serial interface are full-duplex differential pairs of circuits, which can also be combined to increase performance (well, just like PCI Express) into "wide" ports. Accordingly, each device can have more than one port, and each of them can be configured as "narrow" or "wide". Host and expander interfaces can be composed of multiple ports, with the address of each host available to each peripheral, and the bandwidth being added together. Organization of multiple paths for data passage due to the presence of "wide" ports implies parallel execution of commands and a corresponding reduction in the loss of time waiting for a queue.

Conclusion

The presented material is only a brief introduction to the principles of building the architecture of the SAS interface and the implementation features of this standard. A more detailed examination of the interface specifications will most likely require the release of a whole series of articles on this topic. It is possible that this is exactly how it will be, fortunately, the start of mass implementation of the interface is just around the corner, and the number of applied questions on the implementation of SAS systems will only grow over time.

The main definition of SAS, which, in my opinion, should not be forgotten - the new Serial Attached SCSI interface was designed for the needs of a wide range of enterprise-grade storage systems, however, it is still a "close-action" interface and by no means is intended to replace any network interfaces, there is no need to "buy" a similar implementation of the "point-to-point" architecture.

For all its "sharpening" for work in large and almost infinitely scalable storage systems, the Serial Attached SCSI interface implies full compatibility with relatively inexpensive Serial ATA drives, which allows you to design affordable systems even for small businesses. At the same time, support for 2-port Serial Attached SCSI drives allows for performance levels that have never dreamed of today's SCSI-based systems.

For those who are ready to plunge into the study of the features of Serial Attached SCSI on their own, we conclude with a list of sites where educational and standard-setting documents are located.

adaptec website resources
maxtor website resources
seagate website resources

T10:

Serial Attached SCSI -
SCSI Architecture Model - 3 (SAM-3)
SCSI Primary Commands - 3 (SPC-3)
SCSI Block Commands - 2 (SBC-2)
SCSI Stream Commands - 2 (SSC-2)
SCSI Enclosure Services - 2 (SES-2)

SAS Connector Specifications:

SFF 8482 (internal backplane / drive)
SFF 8470 (external 4-wide)
SFF 8223, 8224, 8225 (2.5 ", 3.5", 5.25 "form factors)
SFF 8484 (internal 4-wide)

Serial ATA Specifications:

Serial ATA II: Extensions to Serial ATA 1.0
Serial ATA II: Port Multiplier
Serial ATA II: Port Selector
Serial ATA II: Cables and Connectors Volume 1

Additional resources:

International Committee for Information Technology Standards
T11 (Fiber Channel standards)
SCSI Trade Association
SNIA (Storage Networking Industry Association)

SAS interface.

SAS or Serial Attached SCSI interface provides physical interface connectivity, similar to SATA, devices, sCSI command-driven... Having backward compatible with SATA, it makes it possible to connect via this interface any devices controlled by the SCSI command set - not only hard drives, but also scanners, printers, etc. Compared to SATA, SAS provides a more advanced topology, allowing parallel connection of one device by two or more channels. Bus extenders are also supported, allowing you to connect multiple SAS devices to a single port.

The SAS protocol is developed and maintained by the T10 committee. SAS was designed to communicate with devices such as hard drives, optical drives, and the like. SAS uses a serial interface to work with direct-attached storage devices, compatible with SATA interface. Although SAS uses a serial interface as opposed to the parallel interface used by traditional SCSI, SCSI commands are still used to control SAS devices. Commands (Figure 1) sent to a SCSI device are a sequence of bytes of a specific structure (command descriptor blocks).

Figure: 1.

Some commands are accompanied by an additional "parameter block", which follows the command descriptor block, but is already transmitted as "data".

A typical SAS interface system consists of the following components:

1) Initiators.An initiator is a device that generates service requests for target devices and receives confirmations as requests are fulfilled.

2) Target devices... The target device contains logical blocks and target ports that receive service requests and execute them; after the processing of the request is completed, confirmation of the request is sent to the initiator of the request. The target device can be either a separate hard drive or an entire disk array.

3) Data delivery subsystem... It is part of the I / O system that transfers data between initiators and target devices. Typically, the data delivery subsystem consists of cables that connect the initiator and the target device. Additionally, in addition to cables, the data delivery subsystem can include SAS expanders.

3.1) Expanders. SAS Extenders are devices that are part of the data delivery subsystem and can facilitate the transfer of data between SAS devices, for example, by allowing multiple SAS target devices to be connected to a single initiator port. The expander connection is completely transparent to the target devices.

SAS supports the connection of SATA devices.SAS uses a serial protocol to transfer data between multiple devices, and thus uses fewer signal lines. SAS uses SCSI commands to control and communicate with target devices. The SAS interface uses point-to-point connections - each device is connected to the controller by a dedicated channel. Unlike SCSI, SAS does not require user bus termination. The SCSI interface uses a common bus - all devices are connected to the same bus, and only one device can work with the controller at a time. In SCSI, the transfer rate of information on the different lines that make up the parallel interface may differ. The SAS interface does not have this drawback. SAS supports a very large number of devices, while SCSI supports 8, 16, or 32 devices on the bus. SAS supports high data rates (1.5, 3.0, or 6.0 Gbps). This speed can be achieved by transferring information on each connection, while on the SCSI bus the bus bandwidth is shared between all devices connected to it.

SATA uses the ATA command set and supports hard drives and optical drives, while SAS supports a wider range of devices, including hard drives, scanners, and printers. SATA devices are identified by the port number of the SATA interface controller, while SAS devices are identified by their WWN (World Wide Name) identifiers. SATA (version 1) devices did not support command queues, while SAS devices do support tagged command queues. SATA devices from version 2 support Native Command Queuing (NCQ).

SAS hardware communicates with target devices on several independent lines, which increases the fault tolerance of the system (the SATA interface does not have such an opportunity). At the same time, SATA version 2 uses port duplicators to achieve a similar capability.

SATA is primarily used in non-critical applications such as home computers. The SAS interface, due to its reliability, can be used in mission-critical servers. Error detection and error handling is much better defined in SAS than in SATA. SAS is considered a superset of SATA and does not compete with it.

SAS connectors are much smaller than traditional parallel SCSI connectors, allowing SAS connectors to be used to connect to compact 2.5-inch drives. SAS supports data transfer rates from 3Gb / s to 10 Gb / s. There are several options for SAS connectors:

SFF 8482 - version compatible with the SATA interface connector;

SFF 8484 - internal connector with tightly packed contacts; allows you to connect up to 4 devices;

SFF 8470 - connector with tight packing of contacts for connecting external devices; allows you to connect up to 4 devices;

SFF 8087 - reduced Molex iPASS connector, contains a connector for connecting up to 4 internal devices; Supports 10 Gbps;

SFF 8088 - reduced Molex iPASS connector, contains a connector for connecting up to 4 external devices; Supports 10Gbps speed.

The SFF 8482 connector allows you to connect SATA devices to SAS controllers, eliminating the need to install an additional SATA controller just because you need to connect a DVD burner, for example. Conversely, SAS devices cannot connect to the SATA interface, and a connector is installed on them to prevent them from connecting to the SATA interface.