What is the point of SAS?

Question

Hard disks

What is the meaning of the parallel existence and development of a separate SAS standard when SATA develops and draws inspiration from the same SCSI and why SAS / SCSI-vinci have other capacities (and mysterious multiplicity) and, usually, higher speeds (while the bandwidth limit abilities with the existence of SATA 3 is it kind of like not to fall down)? In what cases, besides brutal high-load, hosting many virtual machines and multi-stream video broadcasting, does it really make sense to install SAS and why?

Sorry if it's a stupid question. Thanks in advance. Interesting.

Answer 1

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 without distortion is written to the device. 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 (such as VLAN, if such an analogy is closer).
- and many many others. I brought only the most well-known features. For those who are interested, read the SAS/SATA
2 specifications. 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.

Answer 2

Plus SAS - not at all in speed. In this matter, they do not differ from SATA. Up to the fact that the disks are almost identical in hardware, and differ only in firmware. And sometimes SATA is even faster if it's an SSD;)
Today, the main difference between SAS is multipath. You can connect a basket (well, or an expander) with disks to not one, but, say, four lines, and the load will be distributed on them, and a line failure (the contact in the connector is bad, for example, or during some work with the server “on a hot "The wire is hooked) will not affect the system's performance - the OS may not even notice the failure, only performance will decrease.
On SATA this is not possible.

Answer 3

SAS only makes sense because of multipath. If it is not needed, then SATA (in larger quantities) for the same money will give more speed.

Answer 4

SAS is SCSI, the SCSI extension is Serial Attached SCSI.
In addition to these cases, it makes sense to use SAS disks, for example, in the case of a DBMS server. DBMS server for hosting, for example, or when storing a massive database.

Answer 5

If you do not go into theory, but purely in practice ... SAS screws on the return of statics (random reading) show about 4 times more bandwidth. Of course, they should also be used for heavy sequential read/write (databases).
The performance of SASok is clearly visible where the amount of data processed by the screw significantly exceeds the amount of server RAM. That is, where the system cache works poorly or does not work at all.

Answer 6

SATA is the little brother of SAS, specially made compatible with the older one. And SAS is the evolution of SCSI, including all the enterprise features that have already been written about here. Moreover, now they cross a hedgehog with a snake - SAS disks for 7.2k are SATA disks (in terms of mechanics and logic), but communicating via SAS, they usually even cost the same.
An even bigger difference in positioning (if SOHO and SMB segments, which make up a small part of the market) is a completely clear division for the enterprise: SAS are disks for combat systems (of any kind), which are always loaded to capacity, and SATA of a large volume is for nearline storage and for backups, that is, a low, irregular load.
By the way, about the volumes - SAS volumes after 72GB and 146GB are multiples of 150GB, probably for the convenience of creating RAIDs, so that vendors can supply disks from different manufacturers trimmed along the same boundaries, and that's all.
In short, the difference becomes clear if you look a little higher at the global market (of which Russia accounts for 5%) and its realities.

Answer 7

SAS approaches SCSI in all performance parameters, unlike SATA
This is very noticeable in databases

Answer 8

Most likely due to marketing division, sas for cool servers, sata for cheap servers and desktops. I don’t see any technical reasons not to make high-speed disks on sata (multipath and expanders are a feature of the controller, into which sata is also inserted).

Answer 9

here is an example of a server using SATA expanders, without any SAS bitblaze.ru/