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Can not find that anywhere it seems.

What is the expected performance difference in a storage backend scenario that is heavily parallellized in access (like a SAN, Virtualization host storage etc.) between SAS and SATA, all other things being equal?

I Think it runs down to the impact of NCQ (32 command limit) to the MUCH higher oustanding command limit of SAS discs.

We are considering replacing some discs and have a chance to go for SAS or SATA - all the rest is in place - and I look for an evaluation from a performance point. Please ignore all the other issues (reliability etc.) - I purely wonder about the impact SAS will have on similar specced discs (RPM etc. being equal). The discs we have in mind can be ordered with both connectors and - there is an idea here to use SATA for ap ossibly repurpose later. THe price difference is not really high, but it made me wonder about the performance impact...

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It's not like you to ask "All things being equal" because the differences in SAS and SATA are enough that things... are not equal. – Mark Henderson Apr 1 '14 at 6:23
@MarkHenderson Reality is - all other things can be equal. Seagate offers some discs with either SAS or SATA connectivity, so all other parameters (cache memory, rotational speed, speed of moving the moving parts) are equal, it all runs down to the SAS / SATA Protocol issues. – TomTom Apr 1 '14 at 6:28
I think you'd have to benchmark the specific disks under your specific workload to be absolutely sure - maybe you can get a few disks of each, benchmark & post it back here? – Andrew Apr 1 '14 at 6:54
@Andrew Yeah, like I am the only one who ever did that. I remember (faintly) once being told to expect about 50% more IOPS based on serious queue lengths being possible in SAS (but not SATA) under heavy parallel load (so a long gueue length does actually happen). Just missing any sort of reference. – TomTom Apr 1 '14 at 7:51
Are you considering nearline SAS at 7.2k RPM, or real SATA? – Basil Apr 3 '14 at 13:45
up vote 9 down vote accepted

Yes, the extensive command set of the SCSI is a big bonus of using it over SATA. from SAS' Wiki:

SATA uses a command set that is based on the parallel ATA command set and then extended beyond that set to include features like native command queuing, hot-plugging, and TRIM. SAS uses the SCSI command set, which includes a wider range of features like error recovery, reservations and block reclamation. Basic ATA has commands only for direct-access storage. However SCSI commands may be tunneled through ATAPI[2] for devices such as CD/DVD drives.

The error recovery commands and block reclamation commands are pivotal in data integrity, S.M.A.R.T. is really for consumer grade equipment.

Also, SAS uses a higher signaling voltage, which enables longer cables compared to that of SATA. That's important when trying to cable up additional storage to an existing SAN.

You menitoned NCQ, but SCSI uses TCQ instead, which can be used in three different modes, however the bigger bonus imo with regard to parallelized setups is the ability to send up to 2^64 commands before filling the queue. Protocols like iSCSI and Fibre Channel limit this right now but the ability is there for future use.

I can only answer that portion, because I don't know if going with SAS for a couple of new disk will give you the same benefit of a purely SAS setup.

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Some of the error recovery features are spilling over into ATA, with some drives now supporting TLER (though I don't believe there is any standardized command in ATA to configure the error timeout, as there is in SCSI). – Chris S Apr 3 '14 at 14:18

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