On Fri, 2008-10-17 at 10:41 -0700, Javier Guerra wrote:
> 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.
Strange that the file system and LVM have enough knowledge of each other
to do online resizing, but not snapshotting.
Guess it wouldn't be easy to implement then.
Kevin
> 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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