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Disabling Disk Write Cache RRS feed

  • Question

  • Hi Everyone,

    I'm just new here. I just would like to ask, how do i disable(uncheck) write caching policy on my VM? every time i uncheck it, it returns that Windows could not change the write-caching setting for the device. Your device might not support this feature or changing the setting.” We have a program that requires us to turn this feature off.

    Thank you.

    Saturday, June 13, 2015 3:46 AM

Answers

  • Hi Sbacalanmo,

    Guest virtual disk cache:
    The virtualized IDE (emulated or synthetic) or SCSI device will report the write cache state that is returned by the lower stack. Virtual disks will report that their write cache is enabled, and they refuse to let the guest turn off the write cache. Disabling the cache will fail and will always respond that the cache is enabled. This behavior is necessary for the following reasons:
    1)Hyper-V cannot make an assumption that all the VMs that are running on the same disk will have to have the disk cache settings be the same.
    2)The underlying storage might have an always-on write cache that cannot be turned off. This is emphasized by the fact that the virtual disk might be migrated to a different disk on the same host (live storage migration) or to a different host (live migration).
     
    Because applications will not be able to turn off disk cache, any application in the guest that has to make sure of data integrity across a power failure will have to use either option 1 or option 2 to make sure that writes bypass the disk cache.

    Refer to:

    Hyper-V storage: Caching layers and implications for data consistency

    Loss of consistency with IDE-attached virtual hard disks when a Hyper-V host server experiences an unplanned restart

    If there is anything else regarding this issue, please feel free to post back.

    Best Regards,

    Anna Wang


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    Tuesday, June 16, 2015 2:33 AM
    Moderator