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Question - Storage caching VMWare ESXi 5.1

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  • Question - Storage caching VMWare ESXi 5.1

    Hi All,

    I need information from you who have more expertise on VM.

    I got 2 (two) identical Dell PowerEdge M620 (each comes with dual SD card, i think each of them is 16 GB, and a 500 GB HDD). Both connected to DELL network SAN storage.

    First machine (say DELL1) is installed with VMWare ESXi 5.1 on the SD card.
    The VMWare ESXi has storage available on local (500 GB) and SAN storage.
    A VM guest machine is set with 1 single virtual disk on the SAN storage (not on local 500 GB).

    DELL2 : Non VM, installed with linux OS on 500 GB HDD.
    Mounted also to same SAN storage.

    I did a writing speed test with following result:
    - DELL1 has better writing speed to SAN storage (its own "local" disk, this is because the virtual disk is actuall on SAN storage), average 240 MB/s.

    - DELL2 surprisingly has slower access speed of 143 MB/s when i tried to write to the SAN storage (on of the mounted directory).

    - Why DELL2 (which is native linux) has poorer writing speed?
    I thought DELL1 would be slower since there is another layer (VMESXi itself) between the linux guest and the SAN storage that can slow the writing to SAN storage.

    - Does VMWare ESXi has a special feature / technology such as caching (so all writing to the SAN actually stores on VMWare cache thus improves the writing speed)?
    Is there any official link that can confirm about this and i can read more ?



  • #2
    Re: Question - Storage caching VMWare ESXi 5.1

    better driver support for the iscsi/fc paths with ESX than with linux ?
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    • #3
      Re: Question - Storage caching VMWare ESXi 5.1

      How are your boxes connected to the storage system?

      Storage systems are pretty much optimised now for VMware and it could possibly be that.

      Isa the Linux system accessing a VMFS LUN? Not a good idea.

      As tehcamel suggests though it could be all down to the drivers that both systems use.


      • #4
        Re: Question - Storage caching VMWare ESXi 5.1

        Seems to be an iSCSI SAN where TS is talking about.
        Might be that mulitpathing is been used on the ESXi environment. Linux might be configured without multipathing.

        Therefore more info is needed.
        Technical Consultant

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        • #5
          Re: Question - Storage caching VMWare ESXi 5.1

          How to ensure multipath is set ?

          I tried this command : "multipath -ll -v2"

          And here is the result:

          "mpathc (36782bcb000155b68000006ac4fa1be5a) dm-0 DELL,MD36xxi
          size=2.5T features='1 queue_if_no_path' hwhandler='1 rdac' wp=rw
          |-+- policy='round-robin 0' prio=6 status=active
          | |- 10:0:0:0 sde 8:64 active ready running
          | `- 8:0:0:0 sdf 8:80 active ready running
          `-+- policy='round-robin 0' prio=1 status=enabled
          |- 7:0:0:0 sdc 8:32 active ghost running
          `- 9:0:0:0 sdd 8:48 active ghost running"

          I think the multipathing is set.

          Back to original question, does anyone know or can confirm if VMware ESXi, by default, uses caching so every i/o request is locally and temporarily stored in the cache before actually read/write to the original storage ?

          Is there any link i can read for more information?

          Thanks so the reply so far guys.


          • #6
            Re: Question - Storage caching VMWare ESXi 5.1

            You'll need to give us more information on how the systems are setup.

            What version of ESXi are you using?

            Are you using vSphere?

            How exactly are you conducting your tests?

            What storage are you connecting to?

            What iSCSI network adaptors are you using?

            Dedicated Hardware or the inbuilt software iSCSI?

            How many ports do you have connected to the SAN on each box?

            What version of Linux are you using?

            From my understanding there is no caching done on the VMware side. I'm trying to find some iSCSI documentation for you to read.


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            Last edited by wullieb1; 1st July 2013, 01:00.