Scsi sas sata comparison. Comparison of SCSI, SAS and SATA interfaces

SAS interface.

The SAS or Serial Attached SCSI interface provides connectivity over a physical interface, similar to SATA, devices, command set-driven SCSI. Possessing backward compatible with SATA, it makes it possible to connect any devices controlled by the SCSI command set via this interface - not only hard drives, but also scanners, printers, etc. Compared to SATA, SAS provides a more developed topology, allowing parallel connection of one device over two or more channels. Bus expanders 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, storage optical discs and the like. SAS uses a serial interface to work with direct-attached drives, compatible with the 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 (Fig. 1) sent to the SCSI device are a sequence of bytes of a certain structure (command descriptor blocks).

Rice. 1.

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

A typical SAS interface system consists of the following components:

1) Initiators. An initiator is a device that originates service requests for target devices and receives acknowledgments as the requests are executed.

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, a confirmation of the request is sent to the initiator of the request. The target device can be either a single 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 may include SAS extenders.

3.1) Expanders. SAS extenders are devices that are part of the data delivery subsystem and make it possible to facilitate data transfers between SAS devices, for example, allowing you to connect several target SAS devices to one port of the initiator. Connecting through an extender is completely transparent to target devices.

SAS supports connecting SATA devices. SAS uses a serial protocol to transfer data between multiple devices and thus uses fewer signal lines. SAS uses SCSI commands to manage 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 the user to terminate the bus. 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 speed of information transfer on different lines that make up a parallel interface can vary. The SAS interface does not have this shortcoming. 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). Such a speed can be achieved by transferring information on each connection, while on the SCSI bus, the bus bandwidth is divided 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 devices (version 1) did not support command queues, while SAS devices support tagged command queues. SATA devices since 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 this capability). At the same time, the SATA version 2 interface uses port duplicators to achieve a similar capability.

SATA is predominantly 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 2.5" compact drives. SAS supports data transfer rates from 3 Gb/s to 10 Gb/s. There are several options for SAS connectors:

SFF 8482 is a variant compatible with the SATA interface connector;

SFF 8484 - internal connector with dense packing of contacts; allows you to connect up to 4 devices;

SFF 8470 - densely packed connector for connection 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 10 Gbps 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 device for recording, for example. DVD discs. 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.

Serial Attached SCSI

Serial Attached SCSI (SAS) - computer interface, designed to communicate with devices such as hard drives and tape drives. SAS uses a serial interface to work with direct attached drives (Eng. Direct Attached Storage (DAS) devices ). SAS is designed to replace the parallel SCSI interface and achieve higher bandwidth than SCSI; at the same time, SAS is backwards compatible with SATA interface: 3Gb/s and 6Gb/s SATA devices can be connected to the SAS controller, but SAS devices cannot be connected to the SATA controller. 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. The SAS protocol is developed and maintained by the T10 committee. The current working version of the SAS specification can be downloaded from his website. SAS supports information transfer at speeds up to 6 Gb / s; transmission speeds are expected to reach 12 Gbps by 2012. With a smaller SAS connector, it provides full two-port connectivity for both 3.5" and 2.5" hard drives (previously only available on 3.5" Fiber Channel hard drives).

Introduction

A typical SAS interface system consists of the following components:

The initiators Initiators) Initiator - a device that generates service requests for target devices and receives confirmations as requests are executed. Most often, the initiator is performed in the form of VLSI. Target devices Targets) The target device contains logical blocks and target ports that receive service requests and execute them; after the processing of the request is completed, a confirmation of the request is sent to the initiator of the request. The target device can be either a single hard drive or an entire disk array. Data delivery subsystem Service Delivery Subsystem) 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 may include SAS extenders. Extenders (expanders) (eng. Expanders) Expanders (expanders) SAS - devices that are part of the data delivery subsystem and make it possible to facilitate data transfer between SAS devices; for example, an expander allows multiple SAS target devices to be connected to a single initiator port. Connecting through an extender is completely transparent to target devices.

Specifications for SAS regulate the physical, data link and logical layers of the interface.

Comparison of SAS and Parallel SCSI

  • SAS uses a serial protocol to transfer data between multiple devices and thus uses fewer signal lines.
  • The SCSI interface uses a common bus. Thus, all devices are connected to the same bus, and only one device can work with the controller at a time. 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 the user to terminate the bus.
  • SCSI has a problem in that the propagation time of the signal on the different lines that make up the parallel interface can vary. The SAS interface does not have this shortcoming.
  • SAS supports a large number of devices (> 16384), while the SCSI interface supports 8, 16, or 32 devices on the bus.
  • SAS provides higher throughput (1.5, 3.0 or 6.0 Gbps). Such bandwidth can be provided on each initiator-target connection, while on a SCSI bus, the bus bandwidth is shared among all devices connected to it.
  • SAS controllers can support connecting devices with SATA interface, when connected directly - using the SATA protocol, when connected via SAS expanders - using tunneling via STP (SATA Tunneled Protocol).
  • SAS, like parallel SCSI, uses SCSI commands to control and communicate with target devices.

Comparison of SAS and SATA

Connectors

As a rule, SAS connectors are much smaller than traditional SCSI connectors, which allows you to use SAS connectors to connect compact 2.5-inch drives.

There are several options for SAS connectors:

  • SFF 8482 is a variant that is mechanically compatible with the SATA interface connector. This makes it possible to connect SATA devices to SAS controllers. Connecting a SAS device to the SATA interface will not work, this is prevented by the absence of a special key cutout in the middle of the connector (see the image of the connector in the table below);
  • SFF 8484 - internal connector with dense packing of contacts; allows you to connect up to 4 devices;
  • SFF 8470 - a densely packed connector for connecting external devices (this type of connector is used in the Infiniband interface, and in addition, it can be used to connect internal 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;
  • SFF 8088 - reduced Molex iPASS connector, contains a connector for connecting up to 4 external devices;
Image code name Also known as External/Internal Number of lines Number of devices A comment
SFF 8482 SAS connector Interior 1 SATA Compatible Form Factor: Allows SATA devices to connect to a SAS controller or SAS connector bar, eliminating the need for an additional SATA controller to connect SATA devices such as DVD recorders. However, SAS hard drives cannot be connected to the SATA bus, because their physical connector has a “key” that does not allow connection to the SATA bus. The connector shown in the figure is the connector on the "disk" side of the interface.
SFF 8484 SAS 4x 32-pin Interior 32 (19) 4 (2) Connector with high contact density; the SFF standard defines connectors for connecting 2 or 4 devices.
SFF 8485 Defines SGPIO (an extension to the SFF 8484 standard), a serial connection commonly used to connect LED indicators.
SFF 8470 Infiniband connector External 32 4 High density external connector (can also be used as an internal connector).
SFF 8087 Internal mini-SAS Interior 4 Molex internal connector
SFF 8088 External mini-SAS External 32 4 Reduced width Molex iPASS external connector for up to 4 devices.

Notes

Links

Computer tires
Basic concepts Address bus Data bus Control bus Bandwidths
Processors BSB FSB DMI HyperTransport QPI
Internal AGP ASUS Media Bus EISA InfiniBand ISA LPC MBus MCA NuBus PCI PCIe PCI-X Q-Bus SBus SMBus VLB VMEbus Zorro III
laptops ExpressCard MXM PC Card
Drives ST-506 ESDI ATA eSATA Fiber Channel HIPPI iSCSI SAS SATA SCSI
Periphery 1-Wire ADB I²C IEEE 1284 (LPT) IEEE 1394 (FireWire) Multibus PS/2 RS-232 RS-485 SPI USB Game port
Universal Futurebus InfiniBand QuickRing SCI RapidIO IEEE-488 Thunderbolt (Light Peak)

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Wikipedia en Francais

Serial Attached SCSI- Sucesor del SCSI paralelo. Aumenta la velocidad y permite la conexion y desconexion en caliente. Al utilizar el mismo conector que serial ATA permite utilizar estos discos, para aplicaciones con menos necesidad de velocidad, ahorrando costes. Los … Enciclopedia Universal

Computer interface for high-speed communication with USB devices storage such as hard drives, solid state drives and flash drives. UAS is dependent on the USB protocol, and uses standard SCSI command sets. Designed for ... ... Wikipedia

Serial ATA- (SATA, also S ATA/Serial Advanced Technology Attachment) Serial ATA Logo Deutsch Wikipedia

Serial Storage Architecture- (SSA) beschreibt eine Methode, um Speichersubsysteme (also Massenspeicher wie Jukeboxen und Disk Arrays) hochperformant an Rechner zu koppeln, insbesondere an Server Systeme oder Großcomputer. SSA ist ein mittlerweile überholter Standard und… … Deutsch Wikipedia eBook


This article will discuss what allows you to connect HDD to the computer, namely, about the interface hard drive. More precisely, about interfaces 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, first things first.

Hard drive interfaces (or, strictly speaking, interfaces external drives, since not only can act as them, but also other types of drives, for example, optical disc drives) are designed to exchange information between these external memory devices and motherboard. Hard drive interfaces, no less than the physical parameters of drives, affect many of the drive's performance and performance. In particular, drive interfaces determine such parameters as the speed of data exchange between hard drive and motherboard, the number of devices that can be connected to the computer, the ability to create disk arrays, the ability to hot-plug, support for NCQ and AHCI technologies, etc. It also depends on the interface of the hard drive which cable, cord or adapter you need to connect it to the motherboard.

SCSI - Small Computer System Interface

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

The appearance of the SCSI interface on the board and the cable connecting to it

The SCSI standard (traditionally, this abbreviation is read in Russian transcription as "skazi") was originally intended for use in personal computers, as evidenced even by the name of the format - Small Computer System Interface, or system interface for small computers. However, it so happened that the storage 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 range of commands, the technical implementation of the interface was rather complicated and was not suitable for the cost of mass PCs.

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

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

SAS - Serial Attached SCSI

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

Serial Attached SCSI was developed as an improvement on a rather old interface connection of rigid Small Computers System Interface drives. Despite the fact that Serial Attached SCSI uses the main advantages of its predecessor, nevertheless, it has many advantages. Among them it is worth noting the following:

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

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

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

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

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

A few 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 - adapter cable from SAS to Serial ATA

Firewire - IEEE 1394

Today, it is quite common to find hard drives with a Firewire interface. Although the Firewire interface can connect any type of peripherals, and it's not a dedicated interface designed exclusively for connecting hard drives, but 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, different from USB, bus for connecting peripheral equipment, primarily multimedia. The specification describing the operation of the Firewire bus is called IEEE 1394.

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

  • Ability to hot connect devices.
  • Open bus architecture.
  • Flexible topology for connecting devices.
  • Widely varying data transfer rate - from 100 to 3200 Mbps.
  • The ability to transfer data between devices without the participation of a computer.
  • Possibility of organization local networks with the help of a tire.
  • Bus power transmission.
  • A large number of connected devices (up to 63).

To connect hard drives (usually through external hard drive cases) 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 known to most users. personal computers. The IDE hard drive interface standard was developed by a 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 the drive controller on the case of the drive itself, and only the host interface adapter for connecting IDE drives remained on the motherboard.

IDE interface on 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 has been reduced. In addition, the installation of an IDE controller inside the hard drive enclosure made it possible to somewhat simplify both motherboards and the production of hard drives themselves, since the technology gave manufacturers freedom in terms of optimal organization of the drive's operation logic.

The new technology was originally called Integrated Drive Electronics. Subsequently, a standard describing it, called ATA, was developed. This name comes from the last part of the name of the PC/AT computer family 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 technology, to the motherboard. Since ATA refers to parallel interfaces (which is why it is also called Parallel ATA or PATA), that is, interfaces that provide simultaneous data transfer over several lines, its data cable has a large number of conductors (usually 40, and in latest versions protocol, it was possible to use an 80-core cable). Regular data cable for this standard has a flat and wide appearance, 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 an IDE cable and a round PATA data cable:

The appearance of the interface cable: on the left - flat, on the right in a round sheath - PATA or IDE.

Due to the relative cheapness of Parallel ATA drives, the ease of implementing an interface on the motherboard, and the ease of installing and configuring PATA devices for the user, drives such as Integrated Drive Electronics ousted devices of other types of interface from the market of hard drives for low-end personal computers for a long time.

However, the PATA standard also has a number of 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 top speed data transmission. Does not support the PATA standard and many advanced features that other types of interfaces have, such as hot plugging devices.

SATA - Serial ATA

View of the SATA interface on the motherboard

The SATA (Serial ATA) interface, as the name suggests, is an improvement on ATA. This improvement consists, first of all, in the conversion of 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 type of data transfer from parallel to serial, the connectors for data transfer and power supply have also changed.

Below is the SATA data cord:

Data cable for SATA interface

This made it possible to use a much 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 connect to the new connectors. True, most new motherboards still have the old connectors and support the connection of old devices. However reverse operation- connecting a new type of drive to an old motherboard usually causes much more problems. For this operation, the user usually requires a Serial ATA to PATA adapter. The power cable adapter usually has a relatively simple design.

Serial ATA to PATA power adapter:

Left general form cable; enlarged on the right appearance PATA and Serial ATA connectors

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

Appearance of a universal bidirectional adapter between SATA - IDE interfaces

At present, 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, which ensured its wide popularity, was support for .

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 ordering read/write operations coming to multiple drives installed in the system. Thus, NCQ can significantly improve the performance of drives, especially hard drive arrays.

Type of adapter from SATA to IDE

NCQ requires technology support from the hard drive as well as the host adapter motherboard. Almost all adapters that support AHCI also support NCQ. In addition, some older proprietary adapters also support NCQ. Also, NCQ requires its support from the operating system to work.

eSATA - External SATA

Separately, it is worth mentioning the eSATA (External SATA) format, which seemed promising at the time, but was not widely used. As you might guess from the name, eSATA is a type of Serial ATA designed to connect exclusively to external drives. The eSATA standard offers most of the features of the standard for external devices, i.e. internal Serial ATA, in particular, the same system of signals and commands and the same high speed.

eSATA connector on a laptop

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

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

Conclusion

The information stored on the hard disk cannot become useful to the user and accessible to application programs until it gets access CPU computer. Hard drive interfaces provide a means of communication between these drives and the motherboard. Today there are many various types interfaces of hard drives, each of which has its own advantages, disadvantages and characteristic features. We hope that the information provided in this article will be useful to the reader in many respects, because the choice of a modern hard drive 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 developed.

Why SAS?

The Serial Attached SCSI interface is not just a serial implementation of the SCSI protocol. It does a lot more than just porting SCSI features like TCQ (Tagged Command Queuing) over the new connector. If we wanted the most simplicity, then we would use the Serial ATA (SATA) interface, which is a simple point-to-point connection between a 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 (expander) and SAS end devices, such as hard drives and host adapters (host bus adapters, HBA). In contrast From SATA, SAS devices can have multiple ports, each of which can use multiple physical connections to provide faster (wider) SAS connections, multiple initiators can access any given target, 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 the connection speed from 3 to 6 Gb / s. This speed boost is very important for complex environments where high performance is required due to high-speed storage. A new version SAS also aims to reduce the complexity of cabling as well as the number of connections per Gb/s of bandwidth by increasing the possible length of cables and improving the performance of expanders (zoning and auto-detection). Below we will talk about these changes in detail.

SAS Speed ​​Up to 6 Gb/s

To bring the benefits of SAS to more wide audience, SCSI Trade Association (SCSI TA) presented a tutorial on SAS technology at the Storage Networking World Conference earlier this year in Orlando, Florida, USA. The so-called SAS Plugfest, which demonstrated 6Gb/s SAS operation, compatibility, and features, took place even earlier in November 2008. LSI and Seagate were the first to introduce 6Gb/s SAS-capable hardware on the market, but other vendors should catch up soon as well. In our article, we'll take a look at the current state of SAS technology and some new devices.

Functions and basics of SAS

Fundamentals of SAS

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 wider 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 that is already connected to the SAS device. Communication can also be carried out through expanders, which act more like switches than like SAS devices.

Features such as zoning now allow administrators to associate specific SAS devices with initiators. This is where the increased throughput of 6Gb/s SAS will come in handy, as a quad-lane 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.

SAS connections are addressed through SAS ports using SSP (Serial SCSI Protocol), but communication at the bottom layer from PHY to PHY is done using one or more physical connections for bandwidth reasons. 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. This results in 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 others work in a similar coding scheme.

SAS and SATA power and data interfaces 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) is closed in the case of SAS, 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 expanders as 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 represented as tree structures like complex Ethernet networks. SAS expanders can work with a large number of SAS devices, but they use the principle of 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 communicate with up to 128 additional SAS addresses. In a SAS 1.1 domain, only two edge expanders can be used. However, a single fanout expander can connect up to 128 edge expanders, greatly increasing the infrastructure capacity of your SAS solution.

Click on the picture to enlarge.

Compared to SATA, the SAS interface can seem complicated: different initiators access the target devices through expanders, which involves laying the appropriate routes. SAS 2.0 simplifies and improves routing.

Keep in mind that SAS does not allow loops or multiple paths. All connections must be point-to-point and exclusive, but the connection architecture itself scales well.

New SAS 2.0 Features: Expanders, Performance


SAS 1.0/1.1
Function Retains legacy SCSI support
Compatible with SATA 		Compatible with 3Gbps
Improved speed and signaling
Zone management
Improved scalability
Storage features RAID 6
Small Form Factor
HPC
High Capacity SAS Drives
Ultra320 SCSI Replacement
Choice: SATA or SAS
Blade servers 		RAS (data protection)
Safety (FDE)
Cluster support
Support for larger topologies
SSD
Virtualization
External storage
4K sector size
Data transfer rate and cable bandwidth 4 x 3Gbps (1.2GB/s) 4 x 6 Gb/s (2.4 GB/s)
cable type Copper Copper
Length of cable 8 m 10 m

Expander zones and automatic configuration

Boundary (edge) and expanding (fanout) expanders practically remained in history. This is often attributed to updates in SAS 2.0, but the reason is actually the 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 different initiators can access storage targets (storages) through the same SAS expander. Domain segmentation is done through zones, access is done in an exclusive way.

For over 20 years, the parallel bus interface has been the most common communication protocol for most digital storage systems. But as the need for bandwidth and system flexibility has grown, the shortcomings of the two most common parallel interface technologies, SCSI and ATA, have become apparent. Lack of compatibility between parallel SCSI and ATA interfaces - different connectors, cables and command sets used - increases the cost of maintaining systems, scientific research and development, training and qualification of new products.

To date, parallel technologies are still satisfactory for users of modern corporate systems in terms of performance, but growing demands for higher speeds, better data transfer integrity, smaller physical sizes, and more standardization are calling into question the parallel interface's ability to cost-effectively keep pace with rapidly growing CPU performance and drive speed. hard drives. In addition, in an austerity environment, it is becoming increasingly difficult for businesses to raise funds to develop and maintain a variety of connectors. rear panels server chassis and external disk arrays, heterogeneous interface compatibility testing, and heterogeneous connections inventory for I/O operations.

The use of parallel interfaces also comes with a number of other problems. Parallel data transmission over a wide stub cable is subject to crosstalk, which can create additional noise and signal errors - to avoid this trap, you have to reduce the signal speed or limit the length of the cable, or both. Termination of parallel signals is also associated with certain difficulties - you have to terminate each line separately, usually the last drive performs this operation 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 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 the 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 transfer them over a cable at speeds up to 30 times faster than parallel interfaces.

SATA expands on the capabilities of traditional ATA technology by enabling data transfer between disk drives at rates of 1.5 GB per second or more. Due to its low cost per gigabyte of disk capacity, SATA will continue to be the dominant disk interface in desktop PCs, entry-level servers and network storage systems, where cost is one of the main considerations.

SAS technology, the successor to the parallel SCSI interface, builds on the proven high functionality of its predecessor and promises to significantly expand the capabilities modern systems enterprise-wide data storage. SAS has a number of advantages that are not available with traditional storage solutions. In particular, SAS allows up to 16,256 devices to be connected to a single port and provides a reliable point-to-point serial connection at speeds up to 3 Gb / s.

In addition, the smaller SAS connector provides full two-port connectivity for both 3.5" and 2.5" hard drives (previously only available on 3.5" Fiber Channel hard drives). This is very useful feature where you need to fit a lot 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 a large number of drives to be connected to one or more host controllers. Each expander provides connections for 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 load balancing.

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

Multiple levels of compatibility

Physical Compatibility

The SAS connector is universal and form factor compatible with SATA. This allows both SAS and SATA drives to be directly connected to a SAS system, thus enabling 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, which provides compatibility between SATA devices and SAS controllers. However, SAS drives cannot work with a SATA controller, so they are provided 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 backplane that supports both SAS and SATA drives. As a result, there is no need to use two different backplates for SCSI and ATA drives. This interoperability benefits both backplate manufacturers and end users by reducing hardware and engineering costs.

Protocol level compatibility

SAS technology includes three types of protocols, each of which is used to transfer data different types via a serial interface, depending on which device is being accessed. The first is the serial SCSI protocol (Serial SCSI Protocol SSP), which transmits SCSI commands, the second is the SCSI Management Protocol (SMP), which transmits 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, combined with the physical compatibility of SAS and SATA connectors, makes SAS technology the universal link between SAS and SATA devices.

Compatibility Benefits

Compatibility between SAS and SATA brings a number of benefits to system designers, builders, and end users.

System designers can use the same backplates, connectors, and cable connections due to SAS and SATA compatibility. Upgrading the system from SATA to SAS is actually a replacement of disk drives. In contrast, for users of traditional parallel interfaces, moving from ATA to SCSI means changing back panels, connectors, cables, and drives. Other cost-effective interoperability benefits of serial technologies include simplified certification and asset management.

VAR resellers and system builders can quickly and easily 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 in choosing the best price/performance ratio 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 become best solution for low-cost servers and storage systems, while SAS drives provide maximum performance, reliability and compatibility with control software. Upgradable from SATA drives to SAS drives without having to purchase new system greatly simplifies the purchasing decision process, protects 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 Working group Serial ATA (SATA) II Working Group announced a collaboration to ensure that SAS technology is compatible with SATA disk drives at the system level.

The collaboration of the two organizations, as well as the joint efforts of storage vendors and standards committees, is aimed at developing even more precise interoperability guidelines that will help system designers, IT professionals and end users implement even more fine tuning of their systems to achieve optimum performance and reliability and reduce the total cost of ownership.

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



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