[PVE-User] qemu write cache mode

Dominik Csapak d.csapak at proxmox.com
Tue Jan 3 13:59:47 CET 2017


On 01/03/2017 11:33 AM, Dhaussy Alexandre wrote:
> Hello,
>
> I just spotted something strange with storage cache mode.
>
> I set cache=none on all my VMs (i believe this is the default), however
> qm monitor says "writeback, direct".
>
> Am i missing something ?
>

from the kvm manpage:

The host page cache can be avoided entirely with cache=none.  This will 
attempt to do disk IO directly to the guest's memory.
            QEMU may still perform an internal copy of the data. Note 
that this is considered a writeback mode and the guest OS must handle
            the disk write cache correctly in order to avoid data 
corruption on host crashes.

so cache=none is considered a writeback mode and this is represented by
writeback, direct

if you select writeback mode for a disk it shows only
writeback

so i think this is correct



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