[Gllug] Back-out plans on Debian/Redhat?

Richard Jones rich at annexia.org
Mon Mar 9 13:30:45 UTC 2009

On Mon, Mar 09, 2009 at 12:43:04AM +0000, Tethys wrote:
> Richard Jones writes:
> >> You can do it live yourself with such fancy tools as tar.
> >
> >Well, not if you want a consistent snapshot
> Just curious... why not? If you use LVM snapshotting (or filesystem
> level snapshotting if you're using an fs that supports that), then
> what are you losing?

LVM snapshots are good.  LVM takes steps to inform the filesystem
layer (sync_fs), block new writers and flush any kernel buffers held
while the snapshot is taking place.  So you'll get consistency similar
to -- actually better than -- typing "sync" and pulling the plug.

At the application level, it completely depends on how the application
is written.  But if some app happens to be updating multiple files at
the time that the snapshot is taken, then there are really no

Tar doesn't give you any guarantees at all.  In fact it's actively
bad.  You get a kind of rolling picture of how the filesystem looked
over as long as it takes to perform the tar.


Richard Jones
Red Hat
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