[pve-devel] Fwd: Re: recommanded cache setting for rbd image
Alexandre DERUMIER
aderumier at odiso.com
Tue Nov 6 19:04:48 CET 2012
>>block: default cache mode is now writeback
>>
>>so it is already 'writeback'?
I think we need to read the qemu code. I don't know if writeback is writeback mode in guest or writeback in host.
cache=none : writeback is guest
cache=writeback : writeback is guest,writeback in host
One another interesting thing in qemu 1.2 changelog:
"Emulated IDE and SCSI as well as virtio-blk devices can now switch the cache mode between writethrough and writeback. virtio-blk automatically switches to writethrough if the guest driver doesn't support flushes. "
So maybe if we set cache=none and that guest doesn't support flushes (drivers,kernel,?), it become writethrough in guest, so it's safe ?
----- Mail original -----
De: "Dietmar Maurer" <dietmar at proxmox.com>
À: "Dietmar Maurer" <dietmar at proxmox.com>, "Alexandre DERUMIER" <aderumier at odiso.com>
Cc: pve-devel at pve.proxmox.com
Envoyé: Mardi 6 Novembre 2012 18:44:34
Objet: RE: [pve-devel] Fwd: Re: recommanded cache setting for rbd image
from the 1.2 changelog:
block: default cache mode is now writeback
so it is already 'writeback'?
> -----Original Message-----
> From: pve-devel-bounces at pve.proxmox.com [mailto:pve-devel-
> bounces at pve.proxmox.com] On Behalf Of Dietmar Maurer
> Sent: Dienstag, 06. November 2012 18:35
> To: Alexandre DERUMIER
> Cc: pve-devel at pve.proxmox.com
> Subject: Re: [pve-devel] Fwd: Re: recommanded cache setting for rbd image
>
> > >>But this is standard behavior, also on physical hosts?
> > Yes, indeed, this is the same with physical hosts. (mainly disk cache).
> > Generaly, if you have a hardware raid controller with battery, the
> > raid controller disable the cache of the disk and use his own cache
> > (so you don't loose data).
> > But if you have single disks with special raid card, you can really
> > loose datas (little, disks have generally some MB of cache)
>
> Data is only pushed to RAID cache if you do an sync - else it can be in RAM
> (no matter if you have BBU or not).
>
> BBU only make sure that sync really makes data persistent.
>
> Or do I still miss something?
>
> > >>I guess we loose much performance with those syncs.
> > yes, we can have a big loose a performance. (or you need a really fast
> > journal
> > (ssd,nvram) on your storage which can handle these syncs)
> >
> > >>Why do we want to be safer than a physical host?
> > I think that user should choose, depend of his applications needs
>
> The user can choose on the GUI, but we need a reasonable default.
>
> Note: this is a really complex issue (most users simply do not understand it,
> and are thus unable to select something reasonable).
>
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