Enterprise Software

2TB SAS drives? All drives are not created equal

IT pro Rick Vanover does a reality check on what may seem like something that is too good to be true -- the 2TB SAS drive. What does that really get you?

If you provision storage for modular systems that can interchangeably accept both SATA and SAS drives, you’ll soon see a dizzying series of options that may make administrators confused. Many drive manufacturers are offering 2TB SAS drives. Yes, that is 2TB SAS. Of course, there is more to it than what it may seem.

My issue with this is that administrators may confuse a SAS drive as better without looking at the details, so I’m trying to ensure that everyone understands the difference in these configurations. In the simplest of configurations, SAS drives may be marked as a "tier 1" storage and SATA as a "tier 2"offering in servers and storage area networks. The traditional SAS drive marketed as an enterprise offering rotates at 15,000 RPM or higher and currently we see 600GB as the largest offering.

I’ve specifically noticed Oracle and HP offering the 2TB SAS drive. The Oracle offering first showed up to me on the Oracle Unified Storage System product line with 2TB SAS drives, and I was a little taken back by this at first. Now, HP is offering 2TB SAS drives that are not much different in price than their 2TB SATA drive. The secret here is that the inner workings of the SAS and comparable SATA drives are effectively the same; the only difference is the bus interface. So, be careful reading specifications that will document a 2TB SAS drive running at 7200 rpm can transfer data at 75 GBps.

The SAS bus can do that, but these drives can’t transfer at that rate. The 2TB SATA drive has a transfer rate of 300 MBps, which I think is more realistic for the SAS version as well. Further, HP’s web site does a good job in spelling out the cost differences between the two models. The SAS version has an MSRP of $949 and SATA version has an MSRP of $899. I’d happily pay the extra $50 for that type of throughput differential, if it were possible.

Of course we should see the good side of this, and that is backplane consistency. The slower drives with a SAS interface can be utilized to ensure a consistent drive type on an array, or if the array doesn’t support SATA drives; the use case can be made for the 2TB SAS drives.

Make no mistake; I’ll still buy the 2TB SAS drive (knowing that it is effectively a SATA drive) for configurations where they are interchangeable. Have you come across this potentially confusing disk configuration? If so, share your comments below.

About

Rick Vanover is a software strategy specialist for Veeam Software, based in Columbus, Ohio. Rick has years of IT experience and focuses on virtualization, Windows-based server administration, and system hardware.

17 comments
cory.schultze
cory.schultze

But I never heard of 75 GB/s data rates! The SAS-2 I know of has a 6 GB/s transfer rate, with: ? Spins of 15k+, giving lower seek-times and faster reads and writes in general ? Twice as fast as SAS and SATA-II interface, 50% faster than FiberChannel ? Reliable data transfer up to 100m for optical cables and 20m for copper cables ? Expander Self Discovery allows for faster discovery of larger topographies and allows for greater arrays And there are NAS solutions available today with >10x 1TB drives with SAS-2 interface.

dwdino
dwdino

The difference is in two words - "Near Line". We now have 3 tiers of storage effectively: SAS, NLSAS, SATA. NLSAS (Near Line SAS) is effectively the combination of a SAS interface with a SATA drive. It provides the large storage volumes needed for file servers and low demand SQL servers while being cost effective and mid-grade performance. We recently purchased many of these drives for our backup operations. When run in a large striped set, the performance is pretty impressive (much better than SATA).

bboyd
bboyd

Is the MTBF up to grade on them?

b4real
b4real

The Spec said 75 GB/s, but the SAS that you and I know is 6 GB/s for the dual port drives. I think it is a difference of backplane and interface, but I clearly see this as FUD.

vendors
vendors

A lot has to do with the controller. If I can read while writing data I am not queueing up data like SATA waiting for one operation to be completed before I can do another. This bi-directional access would definately give me an advantage,

b4real
b4real

Curious if you got to compare, if it is the same drive -> I doubt the SATA interface is slowing the drives down in favor of a SAS interface to the same drive.

b4real
b4real

Too many times, people see SAS and think Enterprise. I am just trying to make sure we all know. Thanks, Alex.

speculatrix
speculatrix

did the author actually know anything about hard drive storage? SATA and SAS share the same physical interface, there are 1.5Gb/s, 3Gb/s and relatively recently 6Gb/s (giga bits per second, not bytes). Many seagate drives have a jumper to set their speed to just 1.5Gbps (retail ones in my experience, if you're not getting the performance you expect, check it out!) for compatibility with older controllers usually on older consumer motherboards. SAS and SATA drives can have any rotational speed but typically SAS, because they are designed for performance, are often 10,000rpm or even 15,000. Electrically sata and sas are, for most purposes, the same. Logically, SATA is a subset of SAS in terms of functions in the drive, so you can stick a SATA or SAS drive on the same bus and expect it to work but only a SAS drive with a SAS controller will achieve the highest throughput. Read the wikipedia entries which are quite good and ignored the rubbish TR article! http://wapedia.mobi/en/Serial_Attached_SCSI http://wapedia.mobi/en/Serial_ATA

EvilJester
EvilJester

Differences I've noted, for the extra $50... the SAS drive has a 3yr warranty, over the one year offered on the Sata. And the SAS has dual port connection, as opposed to the standard single of the Sata. Well worth the little extra in my book. I've been using 1Tb 7200RPM SAS drives, for over 12mths now, in a Dell server, but just as archive drives... low change, less frequently.

Alex Gerulaitis
Alex Gerulaitis

I can't figure out where did 75GBs in HP specs came from... Seems to be neither the drive's nor the interface (dual port SAS 600) spec... The drive should have a transfer rate of 50-60 to 120-130MB/s, the interface - 6Gbs per port... 75GB/s is not even divisible by 6 - so it shouldn't be a controller or a backplane... Anyone has an idea?

DomBenson
DomBenson

The SATA array wouldn't be limited by the bandwidth of the SATA bus, so you're right in that sense. An advantage of the SAS drives is that the SAS spec provides TCQ rather than NCQ which gives the controller more power to influence the drive's behaviour - for example it can expedite data commits that are performed syncronously - while allowing the drive to make head-position based reorderings ot the command queue at other times. NCQ, being either on or off, loses out in at least one of the situations: if it is on, then a vital commit can take much longer to be performed than it needed to, and if off the drive wastes time moving into position. This only really affects moderately concurrent workloads - a simple serial read/write will see no difference on that account.

Alex Gerulaitis
Alex Gerulaitis

what is it in the article that motivated you to questions the author's knowledge or call the article rubbish? His main point is to warn that some large capacity SAS drives don't have the same reliability and performance normally expected from enterprise class SAS drives, and in that, he is right, while your post adds rub... excuse me, very little to it.

b4real
b4real

I would expect them to be the same as the 2TB SATA drives. But, if the warranty is higher just b/c it is SAS -> Then clearly you buy that. But, I don't expect the MTBF to be closer to the 450 GB 15K RPM ENT line of drives.

Alex Gerulaitis
Alex Gerulaitis

The 75GB figure isn't author's, it's HP's. Just follow the links. Anything else?

speculatrix
speculatrix

er, did you read the article and read what I said? author made fundamental errors, things like 75GB/s?

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