Riddle me this: When is a SAS drive not a SAS drive?

Here are two common statement I often hear from clients:

  1. I don’t just want SAS drives, I also want SATA drives.  SATA drives are cheaper than SAS drives.
  2. Nearline SAS drives are just SATA drives with some sort of converter on them.

So is this right?  Is this the actual situation?

First up, if your storage uses a SAS based controller with a SAS backplane, then normally you can plug SAS drives into that enclosure, or you can plug SATA drives into that enclosure.    This is great because when you plug SATA drives into a SAS backplane, you can actually send SCSI commands to the drive plus you can send native SATA commands t00 (which is  handy when you are writing software for RAID array drivers).

But (and this is a big but) what we do know is that equivalent (size and RPM) SAS drives perform better than SATA drives.  This is because:

  • SAS is full-duplex, SATA is simplex.
  • SAS uses the native SCSI command set which has more functionality (which leads to the next point).
  • A SAS drive uses SCSI error checking and reporting which is much more robust than the SATA error reporting.  This allows your storage system to collect richer information from the drive if errors are occurring (such as a failing or marginal disk).
  • SAS drives are dual ported which is vital in dual controller enclosures.

So given a choice (and a very small price differential), why choose SATA over SAS?  SAS is the clear winner.     What we should instead differentiate on is speed (7.2K RPM vs 10K RPM vs 15K RPM vs SSD) and size (2.5″ vs 3.5″ form factor).

Which leads us to Nearline SAS

It is a common belief, that if you buy a Nearline SAS  (or NL-SAS) drive it is really a SATA drive with a SAS connector (interposer) stuck on it.  But this is confusion from the past.

What led to the confusion?

Most midrange and enterprise storage controllers and enclosures up until recent years, used disks that had fibre channel interfaces on them.  We plugged those disks into fibre channel enclosures.  Examples include the DS4700 or the DS8100.  And yet these devices also offered SATA drives.  How did they do this?

They took a SATA drive and added a SATA to Fibre Channel converter card to the disk.   We call this extra piece of hardware an interposer or bridge card.  So people start assuming that this is common practice in every product.  In fact we are now seeing SAS drives being put into  fibre channel disk enclosures by using a SAS to fibre channel interposer.

There are in indeed older products that did take a SATA drive and add a SATA to SAS interposer to achieve a similar thing.   But that really is not necessary any more.  The reason?   The same hard drive can now be ordered from the factory as either a SAS drive or a SATA drive.

Lets look at an example.   If you head over to the Seagate website and look at one of their ranges of 3.5″ Enterprise Drives, you should hopefully make it to this URL:

Seagate have a nice selector tool to let you see all their possible combinations. For instance you can order a  2 TB drive with a 6 Gbps SAS interface, which is a model ST32000444SS:

Or you can order a 2 TB drive with a 6 Gbps SATA interface, which is a model ST2000NM0011:

So what you get is very similar drive hardware (same spindles, heads, motors) but with different adapter hardware, built with the desired adapter at manufacture time. Meaning that if we install this drive into a SAS enclosure, there is no need to add an  interposer or bridge card to the drive after you bought it.

This leads to the next question:

OK.  So this is good, so Nearline SAS drives are MADE as SAS drives.  Does that mean a drive manufactured with a SAS adapter is a SAS drive or a Nearline SAS drive?

Now we are mixing up two different things.  SAS as a standard is a combination of a connection technology (the Serial Attached part) and a command set (the SCSI part). Actually SCSI as a standard also defines both connection methods and command sets.    So SAS is really talking about how we connect to the disk and what command set we use to control the disk.

Nearline on the other hand is a statement about the disks rotational speed and it’s mean time between failure (MBTF).   A Nearline-SAS drive is Nearline because:

  • It rotates slower (7200 RPM)  than the higher specified Enterprise drives (that spin at 10 K or 15K RPM).   Because they are slower they can also hold way more data.
  • It has a lower MBTF (1.2 million hours) than the higher specified Enterprise drives (which are normally specified at 1.6 million hours).

So we have now gone full circle.   A Nearline-SAS drive can use the same physical disk hardware as a SATA drive, but with a superior adapter that uses a superior command set, built onto the drive at manufacture time.

Still confused or want to read some more?  Check out these links:






About Anthony Vandewerdt

I am an IT Professional who lives and works in Melbourne Australia. This blog is totally my own work. It does not represent the views of any corporation. Constructive and useful comments are very very welcome.
This entry was posted in DS8800, IBM Storage, IBM XIV, Storwize V7000 and tagged , , , , , , , , , , , . Bookmark the permalink.

13 Responses to Riddle me this: When is a SAS drive not a SAS drive?

  1. Hi Anthony,

    Excellent post, it’s great to see the public being educated on the differences.

    There are also several major differences between desktop and a near line class hard drives (both SATA and SAS) as pointed out in the links sent by Michael.

    This is primarily around what’s called UER (or UBER) and TLER.

    1. UER or UBER is Unrecoverable Error Rate or Unrecoverable Bit Error Rate and defines the rate or risk of a drive suffer ring a bit flip or media flaws in 10^x

    Desktop Hard drives carry a UER of 10^14 or 1 bit error in 12.5TB transferred, which for a 2TB hard drive if read from end to end will hit at least 1 bit error in 6.25 full transfers (or 6.25 full reads).

    This translates to a 16% risk of data loss per desktop drive or an 80% risk of data loss in a RAID 5-5 during a rebuild (40% if RAID 6 or 1/0).

    However, Near-Line Hard drives carry a UER of 10^15 or 1 bit error in 125TB transferred, which for a 2TB hard drive if read from end to end will hit at least 1 bit in 62.5 full transfers.

    This translates into a 1.6% risk of data loss per Near line dive or 8% in R5-5 (4% in RAID 6 or 1/0)

    Enterprise (10/15k drives) have a UER or 10^16 or 1 bit error in 1250TB transferred, which for mathematical ease, we were to pretend there was a 2TB 15k enterprise drive would translate into 1 bit in 625 full transfers.

    This translates into 0.16% risk of data loss per enterprise drive and 0.8% if R5-5 (if we were to pretend that there was a 2TB enterprise drive).

    Of course, these numbers drop significantly with the additional error checking built into modern arrays though the use of integrity blocks and a variety of other methods.

    2. TLER is Time Limited Error Recovery, this is the time programmed in the drives firmware which limits the time which a drives ECC spends attempting to recover from an error, allowing the RAID controller (in in this case an Array (which is a bit RAID controller really) to acknowledge the error and retry or seek an alternate source of the data requested.
    Most desktop SATA drives have a very long TLER, which in a RAID environment will often cause the array or raid controller to go offline when this is excessively long.

    However, with Near Line SATA and SAS and enterprise drives, this if often programmed by the array vendor to be set to the tolerances of the array, allowing the array’s clever stuff to determine what to do next.

    3. The other bit’s that differentiate them are the RV tolerances, the designed operational hours and servo tracks.

    RV is rotational vibration tolerance, this is the amount of vibration a drive can tolerate before losing drive performance or corruption occurs.

    Desktop SATA drives are never designed to operate in a multi-drive environment, one drive shakes the life out of the other.

    However, NL SATA/SAS and enterprise drives are designed to be in a multidrive environment and use methods such as accelerometers or servo track to detect and compensate for vibration.

    The operational hours are the other factor here, where the bearings and lubricants are often slightly different, with desktop SATA designed to operate for 8 hours a day 5 days a week and NL SATA/SAS and enterprise designed for 24/7/365.

    So you’re right, the drives are often very similar, particularly in appearance, however, in practice, they’re quite different.

    Aus Storage Guy

    • Thanks so much for taking the timing to document that… its almost a blog post in itself!
      I didn’t mention consumer grade disks at all, which I probably should have, as you have clearly explored the differences.
      That is certainly another misconception, that SATA equals Consumer grade versus Enterprise grade.

  2. Hi Anthony,

    Yeah, It’s a bug bear of mine… “why buy that when I can go to [insert retailer of choice] and buy a 2tb drive for $100?”

    It is important to disinguish that a consumer SATA is not the same as a Near-Line SATA and even further removed from NL-SAS, however, this seems to never be articulated well by vendors.

    SATA is just an interface, it doesn’t equate to the quality of the drive as you’ve explained excellently.

    “its almost a blog post in itself!” – Thanks mate, I’m working on that one, happened to have the numbers handy.

  3. Reblogged this on ausstorageguy and commented:
    An excellent post by Aussie Storage Blog on the differences between SATA and SAS.

  4. Pingback: Riddle me this: When is a SAS drive not a SAS drive? « The Storage Tank

  5. Pingback: Riddle me this: When is a SAS drive not a SAS drive? « Storage CH Blog

  6. Pingback: Difference between SAS, SATA and NL-SAS | Home

  7. Dan Lah says:

    Reblogged this on Hoosier Storage Guy and commented:
    Great article here on the Aussie Storage Blog on why NL-SAS > SATA.

  8. Dan Lah says:

    Great article, had no idea on some of the differences between NL-SAS and SATA. This will definitely help me articulate the differences better to customers.

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