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

[Alexandre DERUMIER]

>>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)

>>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

>>Maybe it is needed for complex file formats like qcow2?
Yes, qcow2 is really slow in writethrough (because of metadatas writes). 

>>So we should have a cache default per storage plugin?
why ? Isn't it better for user to choose the cache mode ?

----- Mail original ----- 

De: "Dietmar Maurer" <dietmar at proxmox.com> 
À: "Alexandre DERUMIER" <aderumier at odiso.com>, "Stefan Priebe - Profihost AG" <s.priebe at profihost.ag> 
Cc: pve-devel at pve.proxmox.com 
Envoyé: Mardi 6 Novembre 2012 16:10:42 
Objet: RE: [pve-devel] Fwd: Re: recommanded cache setting for rbd image 

> Yes, it's safe, in the meaning of "you can break your filesystem". 
> 
> But you can loose last x seconds of writes if case of powerfailure. 

But this is standard behavior, also on physical hosts? I guess we loose 
much performance with those syncs. Why do we want to be safer than 
a physical host?