[PVE-User] qemu write cache mode

Dhaussy Alexandre ADhaussy at voyages-sncf.com
Tue Jan 3 14:26:17 CET 2017

Le 03/01/2017 à 13:59, Dominik Csapak a écrit :
> 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
Oh yes, bad for me :(
It is also explained here : https://pve.proxmox.com/wiki/Performance_Tweaks

I think i should have used directsync instead (with SAN storage..)

Thanks for clarifying !

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