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iSCSI or Virtual Disk in Guest OS?

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  • iSCSI or Virtual Disk in Guest OS?

    I'm looking for some advice about configuring storage.

    Given two scenarios - file/print and Exchange - am I better off using the MS iSCSI initiator connecting to individual LUNs (on a MD3000i) or creating one or more LUNs at the host leveland creating virtual disks within those LUNs?

    Is there a significant difference in performance? I've seen one article so far that suggests a slight edge to using the MS initiator directly to iSCSI LUNs. Is there any other confirming or contradictory annecdotal evidence?

    What other Pro/Cons might I see -- in particular, what does the story look like if/when needing to extend the storage?


  • #2
    Re: iSCSI or Virtual Disk in Guest OS?

    You are pondering the thought of using raw iSCSI storage versus encapsulated storage in the form of a VMware .vmdk file. By not virtualizing the disk, you lose some of the benefits inherent in virtualization such as point in time snapshots of the data and encapsulated portability of the VM. You will probably see an increase in performance by using raw storage. The size of the data volume may play a role in your decision. Some people advocate the usage of raw storage for increasingly large volumes. What is large other than an arbitrary number? A terrabyte. Half a terrabyte maybe. Just make sure if you do virtualize the storage that you set your block size large enough to accommodate maximum .vmdk sizes

    block max vmdk
    size size
    1MB 256GB
    2MB 512GB
    4MB 1GB
    8MB 2GB
    VCDX3 #34, VCDX4, VCDX5, VCAP4-DCA #14, VCAP4-DCD #35, VCAP5-DCD, VCPx4, vEXPERTx4, MCSEx3, MCSAx2, MCP, CCAx2, A+ - VMware Virtualization Evangelist
    My advice has no warranties. Follow at your own risk.


    • #3
      Re: iSCSI or Virtual Disk in Guest OS?

      don't forget that
      1. MD3000i (for now) only supports VD size of 2Tb max
      2. MD3000i (if you purchase the feature) is quite capable of snapshots.

      I would keep the OS in vmdk, and the datastores on the SAN.
      Real stupidity always beats Artificial Intelligence (c) Terry Pratchett

      BA (BM), RHCE, MCSE, DCSE, Linux+, Network+