The SAS (Serial Attached SCSI) interface is a serial interface for connecting hard drives, which replaced the parallel SCSI interface. SAS hard drives are designed for use in server systems.
As the name implies, SAS is a relative of SCSI and functionally represents the logical protocol of its predecessor, based on the electrical and mechanical part of the SATA serial interface.
Combined with the new addressing system, this allows up to 128 devices per port and up to 16256 devices per controller.
Currently released SAS controllers and hard drives support data transfer rates of 300MB/s. SAS-2 version devices will transfer data at speeds up to 600MB/s.
History of creation
In 2002, the T10 committee proposed a new SAS protocol that corrected the shortcomings described above. Point-to-point connection provided dedicated bandwidth per drive, cable length limit up to 8m per port (increased with SAS extenders), number of addressable devices per domain increased to 16,256, unique numbers instead of manual ID setting (WWN - World Wide Number), assigned to each of them at the production stage. External SAS connectors support up to four drives and provide 1.2Gbps of bandwidth in one direction. Also in the SAS interface, full support for hot plugging and sorting of the command queue was provided.
SAS standards
The SAS (Serial Attached SCSI) standards set includes:
Application layer: SCSI, ATA, SMP (Serial Management Protocol);
transport layer: SSP (Serial SCSI Protocol), STP (Serial ATA Tunneling Protocol, connecting SATA devices to SAS HBA via expander), SMP (Serial Management Protocol, support for SAS expanders);
SAS port layer
connection level: common part and SSP, STP, SMP;
· SAS phy: speed matching (deceleration by the insertion of fillers); encoding (8b10b as in FC and Ethernet); can be combined into a "wide" (2x, 3x, 4x) port in an HBA/RAID or expander; Speed: SAS-1 - 3Gb/s (300MB/s), SAS-2 - 6Gb/s (600MB/s) ;
· physical layer: full duplex is provided; cables and connectors; a single internal connector is compatible with SATA devices, but not vice versa (SAS devices cannot be connected to a SATA controller); external and group connectors (wide port, several phy); SAS-2 introduced an adaptation period when connecting a device (training, allows you to increase the cable length up to 6m); active cables are introduced into SAS-2.1 (the built-in chip allows to reduce the cable thickness and increase the cable length up to 30m); optical cable - up to 100m; miniSAS x4 connector provides power to the active cable; external miniSAS x4 cables have different connectors for input and output ports; SAS-2.1 adds external miniSAS 8x and internal miniSAS 8x connectors.
Actually, the SAS data transfer protocol means three protocols at once:
1) serial SCSI protocol (Serial SCSI Protocol SSP), which transmits SCSI commands;
2) the SCSI Management Protocol (SMP) control protocol, which transmits control information to the expanders;
3) SATA tunnel protocol (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.
SAS connectors
The SAS connector is universal and form factor compatible with SATA. This allows both SAS and SATA drives to be directly connected to the 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.
The SFF-8482 connector is an internal connector for connecting a standard hot-swappable hard drive with a SAS interface (you can also connect a drive with a SATA interface). Through the connector, in addition to data, power is supplied to the hard drive.
The SFF-8484 connector is an adapter that allows you to connect a backplane or basket with an SFF-8484 connector to the controller. For 2 or 4 devices.
The SFF-8470 connector is a high density external connector. Allows you to connect up to 4 devices. Connector type Infiniband.
SFF-8087 connector - internal mini-SAS connector for connecting up to 4 devices.
SFF-8088 connector - external mini-SAS connector for connecting up to 4 devices
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, optical drives, 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. one.
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 - connector with dense 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 10 Gbps speed.
The 8482 SFF 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.
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. The lack of compatibility between SCSI and ATA parallel interfaces—different connectors, cables, and instruction sets used—increases the cost of system maintenance, research and development, training, and qualification of new products.
To date, parallel technologies are still satisfying users of modern enterprise systems in terms of performance, but the growing need for higher speeds, higher data transmission integrity, reduced physical size, and wider standardization is calling into question the ability of a parallel interface without unnecessary costs to keep up with rapidly growing CPU performance and hard drive speeds. In addition, in an austerity environment, it is becoming increasingly difficult for enterprises to find funds to develop and maintain heterogeneous back panel connectors for server chassis and external disk arrays, verify heterogeneous interface compatibility, and inventory heterogeneous I/O connections.
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, the successor to parallel SCSI, builds on the proven high functionality of its predecessor and promises to greatly expand the capabilities of today's enterprise storage systems. 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 a very useful feature when 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 expense 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 CompatibilityThe SAS connector is universal and form factor compatible with SATA. This allows both SAS and SATA drives to be directly connected to the 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 backplate 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 different types of data over 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 to choose 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 are the best solution for low-cost servers and storage systems, while SAS drives provide maximum performance, reliability and management software compatibility. The ability to upgrade from SATA to SAS drives without having to purchase a new system greatly simplifies the purchasing decision, protects system investment, and lowers 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 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 compatibility guidelines that will help system designers, IT professionals and end users to fine-tune their systems even more to achieve optimal performance. and reliability and lower 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.
Oh, Seagate is not on you;). I saw an excellent presentation about the differences between SAS and SATA from Igor Makarov from Seagate. I try to be brief and to the point.
There are several answers from different angles.
1. In terms of protocols, SAS is a protocol aimed at maximum flexibility, reliability, functionality. I would compare SAS to ECC technology for memory. SAS is with ECC, SATA is without. An example is the following unique features (compared to SATA).
- 2 full duplex ports on SAS devices as opposed to one half duplex on SATA. This makes it possible to build fault-tolerant multi-disk topologies in data storage systems.
- end-to-end data protection T.10. - a set of SAS algorithms that allows using checksums to be sure that the data prepared for recording is written to the device without distortion. And read and transmitted to the host without errors. This unique feature allows you to get rid of the so-called silent errors, that is, when erroneous data is written to the disk, but no one knows about it. Errors can appear at any level. Most often in buffers in RAM during transmission and reception. Silent errors are the scourge of SATA. Some companies claim that on a SATA drive with a capacity of more than 500 GB, the probability of data corruption in at least one sector is close to one.
- we talked about multipassing in previous answers.
- T.10 zoning - allows you to split the SAS domain into zones (of the VLAN type, if such an analogy is closer).
- and many many others. I brought only the most well-known features. Who cares - read the SAS / SATA specifications
2. Not all SAS drives are the same. There are several categories of SAS and SATA.
- so-called. Enterprise SAS - typically 10K or 15K rpm. Volumes up to 1 TB. Used for DBMS and speed-critical applications.
- Nearline SAS - usually 7.2K, volumes from 1 TB. The mechanics of such devices is similar to Enterprise SATA. But still two ports and other charms of SAS. Used in enterprises where large volumes are needed.
- Enterprise SATA, sometimes RAID edition SATA - almost the same as NL SAS, only single port SATA. Slightly cheaper than NL SAS. Volumes from 1 TB
- Desktop SATA - what is put in the PC. The cheapest and lowest quality discs.
The first three categories can be arrayed on controllers from LSI and Adaptec. The last one is absolutely impossible. You won't have problems later. And not because we have a cartel, but because the disks are designed for different tasks. That is 8x5 or 24x7, for example. There is also such a thing as the maximum allowable delay, after which the controller considers the disk to be dead. For desktop drives, it is many times larger. This means that under load, desktop SATA workers will “fall out” of the array.
In short, focus on specific lines for specific tasks. It is best to look at the manufacturers' websites. There are, for example, special low-noise and low-heating screws for home electronics.
The same approaches to SSD, but the area is still not formed, so there are a lot of subtleties. Here we focus on parameters. Although everything that is said in paragraph is true for SSD.
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 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. The new version of SAS also aims to reduce the complexity of cabling as well as the number of connections per Gb/s throughput by increasing the possible length of cables and improving the performance of expanders (zoning and auto-discovery). Below we will talk about these changes in detail.
SAS Speed Up to 6 Gb/s
To bring the benefits of SAS to a wider audience, the 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 over 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 occur 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 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
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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 coding to convert 8 bits of data into 10 character transmissions for 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 similar to 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.
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Compared to SATA, the SAS interface can seem complicated: different initiators access the target devices through expanders, which implies 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.
