13

Well, I was talking with a guy about servers the other day. I was a bit shocked whenever I asked him if there was any significant difference between SCSI and SATA and why he always uses SCSI. (note, I'm not sure if by SCSI he meant SAS)

He told me that SCSI is always faster and that the drives are always more reliable.. I mean, this seems like a bold statement.

He told me something about how SCSI will always be faster than SATA because the OS sends the SCSI (controller?) a request to get a file and it will build the file inside of the SCSI controller, instead of searching all over the disk.. which I do not understand how that would work, so I figure it is BS.

SAS and SATA currently have equivalent data rate speeds..

Is there any true backing for his reasoning that SCSI is always faster and more reliable than SATA?

Earlz
  • 1,059

7 Answers7

10

For the sake of simplicity when I say "SCSI" I'm talking about "traditional" SCSI and SAS

So, on the most basic level SCSI drives will be more reliable simple because they are built better. They are designed and priced to be put into high end machines - primarily servers where they will get a lot of abuse and quite possibly run 24/7/365 for 5-7 years. They are made of higher quality parts, etc. SATA drives are CONSUMER GRADE drives. They are made as cheaply as possible and are not designed with the kind of workloads Servers typically do in mind.

Just as an example, you wouldn't (well some may try but ... ) grab an off the shelf linksys router and try to push 100Mbps of internet traffic from 2000 clients through it.

As far as speed, you will actually see a fairly big difference in speed just do to the spindle speeds of the SCSI devices being so much higher. You're talking 7200 RPM vs 15k RPM on the high ends of both devices. The faster spindle speed, gives you lower latency and higher access speeds. The disk can push data out ... faster.

Deciding on the Drive to put into your machine is a lot more than the theoretical maximum data rate speeds of the interface. With SCSI drives you will get closer to that than with SATA drives.

Dennis Williamson
  • 64,083
Zypher
  • 37,829
4

For SATA, you need to be careful about using a consumer drive if you are building a RAID array.

Some power saving features and in the case of Western Digital, some of their SATA drives have a "deep recovery" process when an error is detected. These can cause a SATA RAID member to be dropped or marked as failed if it is unresponsive beyond the timeout period.

When a SATA RAID5 volume with huge drives drops a member, it is not uncommon for the rebuild to take several hours. During this time, performance will be abysmal.

Western Digital - difference between Desktop edition and RAID (Enterprise) edition hard drives? http://wdc.custhelp.com/cgi-bin/wdc.cfg/php/enduser/std_adp.php?p_faqid=1397

http://en.wikipedia.org/wiki/Time-Limited_Error_Recovery

Greg Askew
  • 39,132
1

Until recently, I would have agreed with the guy and said SCSI (or SAS) is a better solution than SATA. However, today's decent SATA drives are typically faster, more reliable, and cheaper than their SCSI equivalent. Here's an interesting article I read a while back comparing the two ( http://www.westerndigital.com/en/library/sata/2579-001097.pdf ).

Beep beep
  • 1,823
  • 2
  • 18
  • 33
1

  • that thing about the OS asking for the file is totally BS. If that were so, any new filesystem would need firmware upgrades on all drives. SCSI is a block-level protocol. AFAIR, there's not even a 'block-list' read command, just a 'block-range' one. (*)

  • SCSI (and SAS) driver usually are faster and more reliable; but it's just a marketing result: it's the easiest diferentiator, so if you build a 'better' mechanism, put a SAS interface on it and sell for a lot more; build a cheaper mechanism, and put a SATA interface so that it doesn't eat into your higher-end models

(*): there's a feature called 'scatter-gather'; but it's a different thing: it means a controller (SCSI, SATA, network, almost everything these days) can use DMA to read pieces of data from different scattered addresses in RAM, and send it all gathered as a whole.

Javier
  • 9,486
1

For single-user/threaded apps the performance gap is very close but for anything multi-user or multi-threaded SAS/SCSI drives will be faster, often significantly. Also SATA disks will also be less reliable.

This is simply due to design, what they're designed to do when there's just a clean sheet of paper. It's also why expensive german cars are usually faster and more reliable than cheapo cars, extra time, effort and engineering has gone into the SCSI-spec and the build quality of SAS/SCSI disks - that's all, no magic, just effort and cost.

Oh and the thing about files and disks etc. is bullcrap :)

Chopper3
  • 101,808
1

Until very recently, SCSI did tagged command queuing, which means that there could be up to 255 requests outstanding to the drive, which lets the drive optimize their order much better than IDE, which had at most 4 (?).

But now SATA supports it, and I don't think there is that much of a difference. Yes, the SCSI is probably better optimized for multi user/threaded io, but not sure how much that matters.

SCSI is mostly replaced by SAS and arrays of one sort or another. The other issue is, what are your requirements? Space vs speed vs cost? I might go for Solaris on zraid with flash drives for the log for cheap speed.

For a mere 100G, SSD is the way to go, as it is quite affordable, $3K would get you 4 Intel 64GB X25 Enterprise drives, put them in a software RAID10, and they would CRUSH just about any sane combo of rotating drives (SAS, SATA, SCSI, etc).

http://it.anandtech.com/IT/showdoc.aspx?i=3532

"For the rest of us, probably 90% of the market, the Intel X25-E is nothing short of amazing: it offers at least 3 to 13 times better OLTP performance at less than a tenth of the power consumption of the classical SAS drives. We frankly see no reason any more to buy SAS or FC drives for performance critical OLTP databases unless the database sizes are really huge. "

Ronald Pottol
  • 1,743
-4

In the past, you knew you had a SCSI drive because it weighed more. What made them better were higher spindle speed, higher data transfer rate, larger buffer, command queueing, a heavier flywheel and a more powerful motor to spin that sucker.

Now, many of those benefits have been incorporated into SATA and other measures provide spindle stabilization without the heavy flywheel. Even with lower spindle speed, data transfer rates continue to surpass "classic" SCSI on SATA 3.

Of course, with the hindsight available now, SSDs give more bang for the buck at nosebleed speed than any Velociraptor or hybrid drive at a afraction of the power, minimal heat and complete silence.

me_2
  • 1