On Fri, 29 Jun 2012, Brian Edmonds wrote:
> On Fri, Jun 29, 2012 at 2:11 PM, Gregory Farnum <greg@xxxxxxxxxxx> wrote:
> > Well, actually this depends on the filesystem you're using. With
> > btrfs, the OSD will roll back to a consistent state, but you don't
> > know how out-of-date that state is.
>
> Ok, so assuming btrfs, then a single machine failure with a ramdisk
> journal should not result in any data loss, assuming replication is
> working? The cluster would then be at risk of data loss primarily
> from a full power outage. (In practice I'd expect either one machine
> to die, or a power loss to take out all of them, and smaller but
> non-unitary losses would be uncommon.)
Right. From a data-safety perspective ("the cluster said my writes were
safe.. are they?") consider journal loss an OSD failure. If there aren't
other surviving replicas, something may be lost.
>From a recovery perspective, it is a partial failure; not everything was
lost, and recovery will be quick (only recent objects get copied around).
Maybe your application can tolerate that, maybe it can't.
sage
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