[pve-devel] Fwd: Re: recommanded cache setting for rbd image

[Alexandre DERUMIER]

>>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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