On Fri, Oct 17, 2008 at 12:29 PM, Kevin Fox <Kevin.Fox@xxxxxxx> wrote:
> If you were running a single OS with lvm snapshots (IE, no xen), would
> the snapshots be consistent? IE, no fsck needed ever for the snapshots?
> If so, then there must be a mechanism for LVM snapshotting to tell the
> file system to coalesce to disk before the snapshot.
No.
the snapshots ARE consistent. consistent to a point in time where it
was mounted and running. if do an LVM snapshot, and a VM snapshot,
you could recreate the running state.
the difference between a mounted and an unmounted volume is totally
filesystem dependent; it's not the job of a blockdevice layer. (this
is one reason why some people like the "manage everything at
filesystem" approach of ZFS).
that 'coalescing' you refer have two parts: a full sync(), and
unmounting itself. you can do sync() just before doing the snapshot,
so the fsck needed when mounting it would be 'almost' guaranteed to
find it clean. On journalling filesystems, the metadata, at least is
fairly safe. And, on ext3 with data=journal, the files data is just
as safe. Other FSs have similar settings.
--
Javier
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