SAS vs. SATA

[scaryjim]

Could do with a little advice here guys!

I'm speccing new work servers from Dell Premier, and have the option of SATA or SAS Drives. Now, the SAS are considerably more expensive than the SATAs for equivalent capacity, and since I'll be looking at RAID as this is a high availability system that obviously mutlipies the extra cost. My options are basically looking like 2x450GB SAS in RAID1, vs 4x250GB SATA in RAID10. Obviously that gives a little extra storage on the SATA set-up, and it'll cost me £400 less.

So, what are the advantages of SAS over SATA, and is it "worth" the extra money for a production data/web server which will be hosting heavily data-driven websites?

[Lee H]

SAS is full-duplex, SATA is half-duplex.

Systems identify SATA devices by their port number connected to the host bus adapter, while SAS devices are uniquely identified by their World Wide Name (WWN).

SAS protocol supports multiple initiators in a SAS domain, while SATA has no analogous provision.

Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing, each of which has its pros and cons.

SATA follows the ATA command set and thus only supports hard drives and CD/DVD drives. SAS also supports numerous other devices including tape drives, scanners, and printers. However, this advantage could also be moot, as most such devices have also found alternative paths via such buses as USB, IEEE 1394 (FireWire), and Ethernet.

SAS hardware allows multipath I/O to devices while SATA (prior to SATA 3Gb/s) does not. Per specification, SATA 3Gb/s makes use of port multipliers to achieve port expansion. Some port multiplier manufacturers have implemented multipath I/O using port multiplier hardware.

SATA is marketed as a general-purpose successor to parallel ATA and has become more common in the consumer market, whereas the more-expensive SAS targets critical server applications.

SAS error-recovery and error-reporting use SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.

SAS uses higher signaling voltages (800-1600 mV TX, 275-1600 mV RX) than SATA (400-600 mV TX, 325-600 mV RX). The higher voltage offers (among other features) the ability to use SAS in server backplanes.

Because of its higher signaling voltages, SAS can use cables up to 10 m (33 ft) long, SATA has a cable-length limit of 1 m (3 ft).

[aidanjt]

By almost every metric SAS is superior, but unless you're going to get seriously high traffic loads, you're better off with the SATA option. Just use a pair of velociraptors in there instead of those 7,200 rpm disks, that'll keep latency down and if your apache boxen ever does get hammered it'll take the load much better.

[scaryjim]

Thanks for the wealth of information Lee - my head's spinning a bit now!

So the SAS drives should provide significantly higher throughput per drive? What about SAS in RAID1 vs SATA in RAID10 - will the striping I can get with 4x SATA match the throughput of 2x 15k SAS drives without it?

Also I assume, since they're aimed at the enterprise segment, the SAS drives should have a higher MTBF? I'm afraid I can't confirm the exact model / specs of each drive, since I'm having to buy through Dell Premier... ;(

The server load isn't massive currently but it's not light either, and the whole point of buying new servers is that we've got a new, more complex application coming live in the next month or so (which is funding the new servers )...

[aidanjt]

The interface bandwidth isn't exactly the limiting factor to begin with. I don't know if there's any disk which exceeds 200MB/s, much less SATA2.0's 3Gbps.

The main advantage for you is robustness, it's a virtual certainty that the bits written to a SAS disk are exactly the bits the write call made 100% of the time, and the same is true for reads. SATA doesn't provide the same guarantees, but such errors are very very rare.

[badass]

Also, seek times wise the 10,000 Velociraptors are the fastest SATA drives whilst you can get 15,000 RPM SAS drives. Cheap they aren't though. You can't get a 15k drive for much less than £250 for 72 GB. 450 GB are around £450-£500

[scaryjim]

Interesting - for the servers I'm looking at through Dell Premier; adding the 450GB 15k SAS drives cost ~ £370 each inclusive of VAT: sounds like that's a good deal then. I think I'll pony up for 2x 450GB SAS in Raid 1 since I can fit them in the budget

Thanks for the input guys.

[peterb]

The clue is in the name really - Serial Attached Scsi - so you get all the SCSI benefits in a serial transmission protocol. (and AFAIK you can add SATA drives to a SAS controller - although not the other way round - obviously!).. But for a company server, the cost premium is pretty low and worth going for (as I see you have decided that already)

[arthurleung]

The only drawback I see on SAS drive is it is quite difficult to find a SAS controller that DOES NOT do RAID. I had a failed SAS 15K drive which the controller would not detect the JBOD and I had no way of getting data off, or even attempt a recovery because it is SAS.

If it is for a RAID1 it wouldn't be a problem I guess...

[spoon_]

If you go SAS and RAID most definitely you want two arrays on two different controllers, PERC/5i/6i will do.

RAID1 for OS out of two drives and RAID5 out of 4 drives for data.

[scaryjim]

If you go SAS and RAID most definitely you want two arrays on two different controllers ...

Lovely idea, but the budget really can't stretch that far!

[peterb]

There are some SAS tape drives about too, so if yoiu haven't already got a back-up strategy, you could attach one to your SAS cotroller.

http://uk.insight.com/p/TANGA06Q6R/t...ultrium-2.html

[madman045]

Just out of interest, why not 4x300GB SAS?

Is this server going to be running a SQL database along side IIS?

[scaryjim]

Just out of interest, why not 4x300GB SAS?

Cost. 4x300GB ~ £1250, 2x450 ~ £750. It's a tight enough budget that I can't stretch it another £500.

Is this server going to be running a SQL database along side IIS?

It is indeed, but that should basically be it.

There are some SAS tape drives about too, so if yoiu haven't already got a back-up strategy, you could attach one to your SAS controller.

Ooooh, like the look of that. Unfortunately I don't think it'll fit into a 1U server

I'm still pondering backup solutions. Again, the problem is balancing a tight budget and meeting the needs of the service. I'd like some form of tape solution, but I might have to settle for something less ideal

[jim]

Hundreds of USB sticks?

They're definitely portable, and more secure than the tape backup system my work started out with

[scaryjim]

lol

Our existing file server is backed up to a 2.5" USB hard drive which is convenient, was pretty cheap and is easy to lock in a safe. I was meant to have 2 drives to rotate weekly but one got commandeered. I might try that again, but hide the drives so no one else can "borrow" them!