[pve-devel] default cache option=none vs writeback

[Stefan Priebe - Profihost AG]

Hi,

but i think it's still crazy for the users - for me too. An example is
RBD. Default is writeback but rbd still behaves differently if you
select default or if you select writeback.

see for example here:
http://www.spinics.net/lists/ceph-devel/msg06920.html

Stefan
Am 14.02.2013 07:49, schrieb Alexandre DERUMIER:
>>> Maybe we should just display 'default' without '= no cache'? 
> 
> I think we should display default(writeback) or default(nocache), but keep displaying the information.
> Because nobody rtfm the known what is default ;)
> 
> maybe the best way is to follow qemu:
> 
> default(writeback) = no qemu cache option  for all storages. (with no specific .raw, cache=none, trick) 
> 
> That shouldn't change in the futur I think.
> 
> 
>>> AFAIK you get very bad performance with qcow2 files and cache=none 
> 
> Not sure about it, it's true with writethrough/directsync because of the fsync and qcow2 metadatas.
> But it really depends of the physicals storages behind, and their cache implementation.
> 
> My tests my netapp and nexenta seem to show that cache=writeback have a little overhead vs cache=none, 
> mainly when the host need to flush datas.(tested with iscsi, raw, qcow2).
> 
> But some storage like rbd have a good writeback cache feature.
> 
> So it's not easy to auto detect what is the best cache option value.
> 
> 
> 
> So, maybe keep default qemu writeback is a good compromise ?
> 
> ----- Mail original ----- 
> 
> De: "Dietmar Maurer" <dietmar at proxmox.com> 
> À: "Alexandre DERUMIER" <aderumier at odiso.com>, pve-devel at pve.proxmox.com 
> Envoyé: Mercredi 13 Février 2013 16:31:27 
> Objet: RE: [pve-devel] default cache option=none vs writeback 
> 
>> Currently pve-manager display default = no cache, and currently only raw 
>> files are forced with cache=none. 
> 
> Maybe we should just display 'default' without '= no cache'? 
> 
>>
>> but since qemu 1.3, default is writeback. It's very confusing for the users. 
>>
>>
>> So maybe settings cache=none to disk options for all storages as default 
>> could be great ? 
> 
> AFAIK you get very bad performance with qcow2 files and cache=none 
> _______________________________________________
> pve-devel mailing list
> pve-devel at pve.proxmox.com
> http://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel
>