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Asked: 2012-01-16 15:28:38 +0800 CST

RAID10 volume write latency on Windows on ESXi 5

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I have a VMware ESXi 5 environment running on Fujitsu BX922S2 blade servers, connecting to a Fujitsu DX90 SAN.

I've had no problem with RAID5 volumes, but RAID10 volumes (for MSSQL) are exhibiting ridiculous write latency - up over 1000 ms at times, and never lower than about 30 ms. These latency figures are being measured in VMware, using the Performance tab for the SQL server concerned, changing to Advanced, then switching to Datastore view.

The DBA noticed this, and wrote a quick script to continuously create a 1GB file on the server.

Migrating the SQL logs drive to a RAID 5 volume on the same SAN shows no perceptible write latency.

Can anyone point to anything obvious I've configured incorrectly that would cause this?

sql-server storage-area-network vmware-esxi raid10

2 Answers

  1. My SAN experience is with virtual SANs, but to me it sounds like you're down to the following.

    1. You've indentified that the current RAID 5 works fine, but no combination of RAID 0,1 or 5 is working well. You clarified that orignally you were doing a RAID 10 acorss two shleves and that you've tried other simple configurations. What you left out is if you're once again trying these configurations across shelves? I would try a simply RAID 0 on one shelf, and then another one on another shelf. Perhaps its just one shelf that's overwhelmed.
    2. You don't mention if you've tried this with a non-vmware host. One thing I would try is connecting via something else that's outside. This is simply to eliminate a VMware configuration issue.
    3. How does your switching look? Do you have redudant switches, do you have multiple HBA or at least a dual port HBA?
    4. I presume its not a network issue (san wise) only because i would think you'd see a fluxuation in peformance across multiple luns
    5. Do you have multi pathing enabled in vmware? I suspect fujitsu has an expension pack for this.
    6. If it was me, I would call the SAN vendor and ask why it might be slow. Perhaps there's something obvious to them.
    7. BTW, i assume this is all fiber channel?
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  2. Figure out whether you can pin the write latency issues down to a specific controller. Check all your cabling. Check your FC/iSCSI switch for errors on a specific port. Try testing without MPIO enabled. Try testing writing directly to the volume outside of VMware.

    After that, make the manufacturer figure it out.. this is definitely their problem to solve.

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