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

[Dietmar Maurer]

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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