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

[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