Modern computer systems use SATA and SAS interfaces to connect main hard drives. As a rule, the first option suits home workstations, the second - server ones, so the technologies do not compete with each other, meeting different requirements. The significant difference in cost and storage space leaves users wondering how SAS differs from SATA and looking for trade-offs. Let's see if it makes sense.
SAS (Serial Attached SCSI) is a serial storage device interface designed on the basis of parallel SCSI to execute the same instruction set... Used primarily in server systems.
SATA (Serial ATA) is a serial data exchange interface based on parallel PATA (IDE). It is used in home, office, multimedia PCs and laptops.
If we talk about HDD, then, despite the differing specifications and connectors, there are no cardinal differences between the devices. Backward one-sided compatibility makes it possible to connect disks to the server board both one by one and via the second interface.
It is worth noting that both connection options are real for SSDs, but the significant difference between SAS and SATA in this case will be in the cost of the drive: the first one can be ten times more expensive with a comparable volume. Therefore, today such a solution, if not uncommon, then it is sufficiently balanced, and is intended for fast corporate-level data centers.
As we already know, SAS is used in servers, SATA - in home systems. In practice, this means that many users simultaneously access the former and solve many problems, while the latter is dealt with by one person. Accordingly, the server load is much higher, so the disks must be sufficiently fault-tolerant and fast. The SCSI protocols (SSP, SMP, STP) implemented in SAS allow more I / O to be processed concurrently.
Directly for the HDD, the access speed is determined primarily by the spindle rotation speed. For desktop systems and laptops, 5400 - 7200 RPM is necessary and sufficient. Accordingly, it is almost impossible to find a SATA drive with 10,000 RPM (unless you look at the WD VelociRaptor series, designed, again, for workstations), and everything above is absolutely unattainable. SAS HDD spins at least 7200 RPM, 10000 RPM can be considered a standard, and 15000 RPM is a sufficient maximum.
Serial SCSI drives are considered to be more reliable and have higher MTBF. In practice, more stability is achieved through the checksum function. SATA drives, on the other hand, suffer from "silent errors" when data is partially written or damaged, which leads to the appearance of bad sectors.
The main advantage of SAS - two duplex ports, allowing one device to be connected via two channels - also works for the system's fault tolerance. In this case, the exchange of information will be carried out simultaneously in both directions, and reliability is ensured by the Multipath I / O technology (two controllers insure each other and share the load). The queue of tagged commands is up to 256 deep. Most SATA drives have one half-duplex port, and the NCQ queue depth is no more than 32.
The SAS interface assumes the use of cables up to 10 m long. up to 255 devices. SATA is limited to 1m (2m for eSATA), and only supports one point-to-point connection.
Prospects for further development - what the difference between SAS and SATA is also felt quite sharply. The SAS interface reaches 12 Gbps throughput, and manufacturers are announcing support for 24 Gbps data rates. The latest revision of SATA stopped at 6 Gb / s and will not evolve in this regard.
SATA drives have a very attractive price tag in terms of the cost of 1 GB. In systems where the speed of data access is not critical, and the amount of stored information is large, it is advisable to use them.
|For server systems||Mainly for desktop and mobile systems|
|Uses SCSI command set||Uses ATA command set|
|Minimum spindle speed HDD 7200 RPM, maximum 15000 RPM||Minimum 5400 RPM, maximum 7200 RPM|
|data records||High error rate and bad sectors|
|Two duplex ports||One half duplex port|
|Supported Multipath I / O||Point-to-Point|
|Command queue up to 256||Command queue up to 32|
|Cables up to 10 m can be used||Cable lengths up to 1 m|
|Bus bandwidth up to 12 Gbps (per prospect - 24 Gb / s)||Bandwidth 6 Gb / s (SATA III)|
|The cost of drives is higher, sometimes significantly||Cheaper in terms of at the price for 1 GB|