Serial Attached SCSI
Serial Attached SCSI ( SAS ) is a computer interfacethat succeeded the previous parallel SCSI interface in2004.
background
The forerunner of SAS was the parallel SCSI interface, which had reached physical limits with its last standard, Ultra-320 SCSI. In the case of parallel transmission, among other things, the signal propagation time on the individual data lines must not differ too much so that the slowest and fastest bit can still be evaluated at the bit sampling point. A new serial interface was therefore designed to further increase the data throughput rate. Since the serial interface Serial ATA (SATA) had already been introduced in desktop PCs a few years earlier , it made sense to provide the SCSI successor with a mode compatible with SATA in order to reduce development and manufacturing costs. Ultra-640 SCSI was defined as a parallel standard, but no more such devices were built.
technology
On the physical layer, SAS works serially with a symbol rate of 3 Gbit / s (SAS-1, 2004), 6 Gbit / s (SAS-2, 2009) or 12 Gbit / s (SAS-3, 2013). A fourth generation with 22.5 Gbit / s, slightly more efficient coding and integrated forward error correction was released in 2017. Up to and including SAS-3 is used as a modulation on the data link layer a as in SATA 8b10b code used, i. H. 8 bits of user data are transmitted in 10 channel bits . Compared to SATA, a larger voltage swing (± 1.2 V) is used on the cables to ensure a range of up to 6 m. A 128b / 150b encoding is used for SAS-4. As a result, the specified transfer rates correspond to 300, 600, 1200 or 2400 MB / s net data throughput.
| generation ration |
Surname | Symbol rate |
Data throughput (net) |
Coding | appearance year |
|---|---|---|---|---|---|
| SAS-1 | SAS | 3 Gbit / s | 300 MB / s | 8b / 10b | 2004 |
| SAS-2 | SAS 6G | 6 Gbit / s | 600 MB / s | 2009 | |
| SAS-3 | SAS 12G | 12 Gbit / s | 1200 MB / s | 2013 | |
| SAS-4 | SAS 24G | 22.5 Gbit / s | 2400 MB / s | 128b / 150b | 2017 |
| SAS-5 | SAS 48G | 45 Gbit / s | 4800 MB / s | in development |
In contrast to its predecessor, Parallel SCSI, SAS is a point-to-point connection. On the one hand , the terminators typical of parallel SCSI , which avoid signal reflection at the cable ends, are no longer necessary. On the other hand, there is no need to manually assign SCSI IDs to the drives: each SAS device has a globally unique address.
The SAS standard provides three protocols higher up in the protocol stack to enable device communication:
- SAS SCSI Protocol (SSP), the serial variant of the SCSI protocol. SAS end devices and SAS controllers use this to communicate with each other.
- Serial ATA Tunneling Protocol (STP), which tunnels the SATA protocol. This makes it possible to use SATA drives on SAS controllers.
- SAS Management Protocol (SMP), for the management of expanders.
expander
SAS is not only designed as a connection protocol between the controller and end devices, but also allows a storage domain to be set up from SAS devices with the help of so-called "expanders". These devices have a similar function to the switches known from the network world . A distinction is made between two types of expander:
- The Edge Expander theoretically bundles up to 128 end devices (practically available up to 36) on a single SAS cable. By connecting this to the controller, it is possible to operate significantly more devices on a controller than this port offers. Up to two edge expanders can be used per controller.
- If the options extended by Edge Expander are not sufficient, Fanout Expander come into play. These can be placed between the controller and the edge expander and fan out the individual controller connection to up to 128 ( fan out ). Using both types of expanders increases the theoretical maximum number of end devices to 128 × 128 = 16384 drives.
Line bundling and dual porting
SAS hard drives usually have two SAS connectors. These can either be bundled to increase performance so that together they can transmit up to 24 Gbit / s (third generation). Or you can use it for dual porting , which allows the two ports of the hard drive to be connected to different host adapters. This makes it easy to implement redundant architectures. At the Intel Developer Forum in February 2004, Seagate demonstrated a 2.5-inch SAS hard drive in dual-porting mode. During the demonstration, two independent systems accessed the hard disk used , each of which played a video stream from this disk.
Connector and cable
SAS takes on the SATA plug connections in a slightly modified form: the sockets are provided with a bar between the data and power connection, the plugs with a wedge accordingly. This means that SATA cables cannot be plugged into SAS devices, but SAS cables can be plugged into SATA devices. This property was deliberately developed due to the planned backward compatibility with SATA.
In the course of time, various connector types have become established that are specified in several SFF standards:
| image | Surname | other names | external / internal |
Number of connectors |
equipment | Remarks |
|---|---|---|---|---|---|---|
|
SFF-8482 | internally | 7th | 1 | For internal connections, compatible with SATA drives. The picture shows the connector on the drive side. |
|
|
SFF-8484 | internally | 32 (19) | 4 (2) | For two or four-channel internal connections. | |
| SFF-8485 | Defines SGPIO (extension of SFF 8484), a serial protocol. For example used for LED displays. |
|||||
|
SFF-8470 | InfiniBand connector | external | 32 | 4th | For four-channel external connections, also used by InfiniBand . Also used as an internal connector. |
|
SFF-8087 | internal mini-SAS, internal mSAS, iPASS (Supermicro) |
internally | 36 | 4th |
Molex connector, for four-channel internal connections. Much smaller than SFF 8484, also for 12 Gbit / s. |
|
SFF-8088 | external mini-SAS, external mSAS |
external | 26th | 4th |
Molex connector for four-channel external connections, also for 12 Gbit / s. The number / position of the grooves on the plug can differ in order to determine the plug direction ("IN" or "OUT"). Three-groove plugs can be used for IN and OUT. |
|
SFF-8643 | internal mini-SAS HD | internally | 36 | 4th | Connector for four-channel internal connections, also for 12 Gbit / s, also from the controller to drives as fan-out. Again smaller than SFF-8087. |
|
SFF-8644 | external mini-SAS HD | external | 4th | Connector for four-channel, external connections, also for 12 Gbit / s |
Comparison between SAS and SATA
| interface | SAS (SAS-3, third generation) |
SATA (Serial ATA 6.0 Gbit / s, SATA revision 3.x) |
|---|---|---|
| Transmission rate per port | 12 GHz symbol rate ≙ 1200 MB / s gross data rate | 6 GHz symbol rate ≙ 600 MB / s gross data rate |
| Line code | 8b10b code | 8b10b code |
| Voltage swing (Tx / Rx) | 800-1600 mV / 275-1600 mV | 600-900 mV / 325-900 mV |
| Impedance | 100 Ω | 100 Ω |
| Connection type | full duplex | half duplex (full duplex not supported) |
| Line bundling | Port aggregation | No |
| Dual channel | For hard disk / SSD 2 × 12 GHz symbol rate ≙ 2400 MB / s gross data rate |
No |
| Connection technology | Multi-initiator | Single host or multilane |
Hard disk connector on the bottom
Hard disk connector on top
SAS backplane
SFF-8484 connector
Web links
- Official SAS website of the SCSI Trade Association
- SAS connector specifications
- Progressive connector solutions unlock the potential of SAS-1200 - workshop on Storage-Insider.de
- T10 Technical Committee (responsible for the SAS standard)
Individual evidence
- ↑ a b SAS - Serial Attached SCSI. Accessed April 30, 2020 .
- ↑ a b SAS Protocol Layer - 4 (SPL-4) draft 08b; 5.5 SPL packet (PDF) INCITS. July 18, 2016. Retrieved July 21, 2016.
- ↑ Serial Attached SCSI - Thomas-Krenn-Wiki. Accessed April 30, 2020 .
- ↑ t10.org (PDF)
- ↑ snia.org
- ↑ SAS Integrators Guide . SCSITA. April 21, 2006. Retrieved September 29, 2017.
- ↑ tomshardware.com