Thursday 28 April 2011

SAS versus SATA and Hardware RAID versus Software RAID

In the last few years we have made some changes to the server configurations that we either build and deploy or Tier 1 provides:

  • We install SAS 10K or SAS 15K drives over SATA.
    • Performance for one is vastly superior on the SAS drives.
    • SAS drives are a lot more sensitive to bad sector behaviour and are better able to recover from bad data being tossed up.
    • SAS drives use many forms of ECC which protects the integrity of the data.
  • We install a hardware RAID controller with battery backup or SSD Cache over using the on board software RAID.
    • The on board “RAID” is software driven. All RAID calculations are completed by the server’s CPU and in many cases require the software driver to rebuild or function properly – meaning we need to boot into the OS.
    • We have had difficulties with on board software RAID recoveries and ShadowProtect due to driver issues.
    • Hardware RAID on Chip with the battery backup or SSD Cache virtually eliminates the parity write cost of RAID 5 and greatly reduces that cost for RAID 6. SSD Cache almost renders the whole discussion moot as the most frequently requested data sits on the SSD.
    • Hardware RAID controllers have the ability to mitigate the failure of a drive by keeping the server up. In our experience software RAID tends to freeze the box if a drive fails.
    • Hardware RAID controllers tied to SAS drives have a much better chance of mitigating or eliminating the possibility of data corruption if sectors on an array member are dying.

Intel has a couple of resources for the above discussion as do many other RAID related vendors.

One Intel resource is the following Intel support page: Intel Server Products: Choosing between SAS vs. SATA Hard Disk for your Server RAID System.

There we find this grid that gives us a pretty good idea of some of the significant differences between SAS and SATA drives:


We are given an extensive explanation in the following document which is linked to below the above table: Intel – Enterprise-class versus Desktop-class Hard Drives (Link to PDF document download).

We put our server configurations through a lot of testing before we deploy them to client sites or within our own organization. We do this because we want to make absolutely sure that the server configuration we are going to deploy will meet the needs of our client over the life of the box which is about 36 months.

The extra cost for the hardware RAID controller, battery backup, and 15K SAS drives (not much of a cost difference between 300GB 15K SAS and Seagate Enterprise SATA these days) when taken over the life of the box (divide that cost by 36) is actually quite small relative to the performance, data protection, and overall storage stability benefits.

Tier 1 Caveat

Keep in mind that cost inferences mentioned here are for our in-house Intel server solution components. When it comes to Tier 1 the costs of some server components and server storage can be extremely high relative to components supplied in our solutions.

Philip Elder
Microsoft Small Business Specialists
Co-Author: SBS 2008 Blueprint Book

*Our original iMac was stolen (previous blog post). We now have a new MacBook Pro courtesy of Vlad Mazek, owner of OWN.

Windows Live Writer


Anonymous said...

This was one of the very (stress very) few things I did not agree with your setups in the past. The use of SATA drives which always made me wince.

I have done it myself years ago and still kick myself for at least 2 failures that cost us face.

Nice to see your SASSY moves.

Philip Elder Cluster MVP said...


Yeah, it took a while for me "to see the light"! ;)

We have had some funky stuff happening on the SATA side for a while now that we just do not see on servers configured with SAS drives.

Thanks for the comment,


Anonymous said...

How about the newer Nearline SAS drives such as the Seagate Constellation ES's? Supposedly the bridge between the two.
Perhaps they have a place in simple file servers or something without a lot of IO, but only time will tell.

Philip Elder Cluster MVP said...


For light I/O needs and nearline storage needs we have been looking to the new SAS lines.

We will be running some tests on these drives for iSCSI target usage for local backups as well as general aggregate storage.


Anonymous said...

We've always gone for SAS, apart from very small clients (5 or less) for whom an ML110 with SBS seems to fit quite well.

We do often put a single 500gb sata drive to complement the SAS arrays for bulk storage where tolerance and speed aren't really required, just volume - backup cache, WSUS updates etc, as for things like this the cost is hard to justify.