Hidden GUI disk policy

Whilst reviewing the new ESX4 Web Admin Guide last night, I came across a “new feature”.  If you log into a ESX4 WebAccess session and add a new disk to a VM, you have the option to change the “Write caching” policy from the GUI.  This option isn’t available from the vClient view.

ESX4 disk policy

After a bit of investigation, if you go with the “Optimize for Safety” option (the default), it adds the line scsi0:1.writeThrough = “TRUE” in the vmx file.  If you select the “Optimize for Performance”, then it omits this line.  Interestingly if you use the vClient to add a disk, it doesn’t add this line.

This means that by default, adding a disk via ESX4 webAccess produces different results than doing it via the vClient. I suspect this is an option which was removed from the vClient, but they forgot to remove it from the webAccess.

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