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Disable write cache on ESX 2.5 virtual machine

Posted on 2008-06-25
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Last Modified: 2010-04-21
I have a virtual machine (W2K3 SP2 + SQL server) running on an ESX 2.54 host. I want to disable the write cache on the disk which contains the SQL database.
The option which I normally use in the Windows OS is set to "Optimize for performance" and is grayed out to prevent change.
How can I disable the write cache.

Any help is very much appriciated!

Rob
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Question by:robv73
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by:agriesser
ID: 21864886
From the virtual machine side, set the disk type to be independent/persistent, that way, all the changes are permanently and immediately written to the disk.

If you want to disable it inside the guest, you need to modify your .vmx file and probably reboot the guest too:

diskLib.dataCacheMaxSize = "0"
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Author Comment

by:robv73
ID: 21865591
I added this line to the VM config (vmx) file and restarted the VM.
But when I check the "Policies" tab of the disk properties. The option is still set to "Optimize for performance"......which should mean: write cache enabled...

Is this ok?!
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agriesser earned 500 total points
ID: 21865798
According to some forum entries at vmware.com, it shouldn't matter wheter this option is greyed out or not. Windows asks the SCSI disk driver for its features and if the driver (in this case the Virtual VmSCSI driver) does not support write caching on a per-hardware base (which it simply can't support), the option in windows is greyed out. Telling the disklib to not cache data at all should do the trick although it might seem to still be disabled inside the VM.

I don't know how to test if that works, do you see any difference with this option being set?
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by:robv73
ID: 31470511
Thanks for your quick response / help !
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