Raid10 - benefits of replacing SATA drives with SAS

Posted on 2016-10-26
Last Modified: 2016-11-22
Dell poweredge R420 with Perc 710 controller.  Currently RAID10 with 4 2TB WD NAS SATA drives.  No problems other than we'd like to see some more RAID throughput.  This server is used as an iSCSI SAN for a VM environment - OS is Windows 2012 R2 - running Starwind SAN.   I have seen where IOPS are nominally twice as high with SAS compared to SATA.  When I built the server a few years ago SAS drives were still pricey - but now a 2TB drive is $100+.  So - will I see a decent speed increase in the iSCSI transfers?  If so, can I just replace these drives one at a time, letting the array rebuild between replacements?  Or will the PERC complain if I mix SAS and SATA (temporarily).   Or is there any way to do an image copy of each array member SATA drive to a new SAS drive and then fire it up with all SAS?  Intuitively this seems like it will work but likely there are geometry differences and I know that can screw up RAID.

I don't have any place to backup the entire SAN partition anywhere - at 3.8 TB it's too big.  I would have to backup individual VMs and reload each one after restoring the OS partition. Obviously replacing one drive at a time would be MUCH simpler.
Question by:dvanaken
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Assisted Solution

andyalder earned 250 total points
ID: 41860839
>I have seen where IOPS are nominally twice as high with SAS compared to SATA

With both being 7.2K or were the SAS ones 10 or 15K?

>can I just replace these drives one at a time?
No, because you can't have SAS and SATA in the same array. The PERC won't use the SAS disk as a replacement for a failed SATA.

Author Comment

ID: 41860892
I can't say that I recall the rotational speeds I read about.  I do know that the current SATA drives are 5400 (WD20EFRX) and I am considering using Seagate Constellation 7200 SAS drives (ST2000NM0023).  

Well - in looking up specs I found a site which compared them and liked the SATA drive better for perfomance: The 2 TB (WD20EFRX) and 3 TB (WD30EFRX) models manage to achieve a sequential read speed of 112 MB/s. This is faster than the other 5400 RPM drives. They even beat the 7200 RPM Seagate Constellation ES.

So I guess I answered my question with respect to this replacement candidate.  The same website showed the Seagate Barracuda to have much higher read transfer rate (154 MB/s vs 112 MB/s for the current drive I am using).   They're SATA - so I guess that's one simple solution...
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Accepted Solution

dlethe earned 250 total points
ID: 41861042
Those benchmarks aren't applicable to real-world server use.   SAS drives are dual-ported, have much higher queue depth, so that means not only a fatter pipe, but lower latency.  

Not only that, but the 2 disks you mentioned are advanced sector (4K) drives.  That means all I/O is in chunks of 4KB.  The SEAG disk is not advanced sector, so  you aren't doing fair comparison.

Those WD drives are hardly server / multiuser class designed for typical multiuser multithreaded I/O.  Those are workstation drives.   Go with SAS if you care about running real-world loads, and not artificial benchmarks.

Author Comment

ID: 41861067
Thanks for all the help.
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Author Closing Comment

ID: 41861070
I appreciate the help.  I am going to leave things for now and look into next year's budget to pick up some higher performance drives.
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Expert Comment

ID: 41861080
Sequential performance is meaningless for a SAN in a virtual environment, the I/O is random not sequential unless you just have one VM which stores videos. You need to compare the IOPS of the drives.

Expert Comment

by:Taras Shved
ID: 41865150
There are couple of options for you to speed up your iSCSI based storage server:
1)      As you’ve already mentioned, replacing current slow SATA drives with SAS ones is a good idea. Unfortunately, you can not upgrade disks one by one letting the RAID array to rebuild after each replacement. But you can clone old drives to new ones using Norton Ghost or Acronis and another PC or Server one by one and then let the RAID controller to import foreign configuration. That might work, still requires downtime and a bit of time, though.
2)       Since you are a Starwind Virtual SAN user you can try using log-structured file system devices instead of traditional one. These were specially designed to sequentialize random workloads speeding up even slow SATA spindle drives, so you definitely should give it a try.
3)      Another good option might be adding an SSD drive to your existing server if possible and enabling level 2 flash-based caching on Starwind side. As soon as your “working set” (or hot data) is fully or partially covered by the SSD caching, your performance will be significantly better.
Hope it helps.

Author Comment

ID: 41866161
Taras-  Thank you for those suggestions.  I have been  thinking of adding an SSD to the SAN but LSFS is new to me.  I read the starwind link you sent and it makes great sense but I don't see how to implement this.  Is there a set of directions somewhere?   Thank you.

Expert Comment

by:Taras Shved
ID: 41867643
Sure @dvanaken, here you go

Should you have any questions do not hesitate to contact me or guys on StarWind's forum. They are very friendly and helpful.

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