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

[Dietmar Maurer]

> >>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).