[linux-lvm] LVM Snapshots & journal recovery
Heinz J. Mauelshagen
Mauelshagen at sistina.com
Mon May 28 10:43:03 UTC 2001
On Wed, May 23, 2001 at 08:48:42AM -0600, Peter J. Braam wrote:
>
> Joe,
>
> Thanks for explaining this.
>
> That's still not good enough, but better: you must also wait for the
> redirection table to be stable on disk before you can overwrite the origin
> device.
>
> So journal recovery can indeed work if LVM snapshots wait twice:
>
> - first for the copied disk to become stable on disk
> - then for the redirection to become stable on disk
>
> at that point one can safely over-write the origin.
>
> First, looking at the code, I don't think that this is happening at all.
Well, it does.
> Second, to have these two extra waits seems really quite a bad penalty to
> me.
Sure it is if you just argue from that standpoint.
OTOH it is a clean approach to have the snapshot data and metadata consistent
*before* any update to the original data takes place at all.
The design of the LVM so far was going after insuring eactly that in order
to ensure data integrity for *any* user of such LVM blockdevices and not just
journaling filesystems.
But BTW: asynchronous snapshot writes are already on our post 1.0
schedule anyway (likely choosable) :-)
Regards,
Heinz -- The LVM Guy --
>
> - Peter -
>
>
> On Wed, 23 May 2001, Joe Thornber wrote:
>
> > On Tue, May 22, 2001 at 03:32:19PM -0600, Peter J. Braam wrote:
> > <snip>
> > >
> > > But during recovery, LVM cannot possibly know if the whole process of
> > > copying out the data from the current to the snapshot area completed
> > > during the previous run. Yes, LVM updates the redirection table first and
> > > then copies, but, still, you don't know _where exactly_ the writes stopped
> > > happening and in particular you don't know if the block was copied already
> > > or not.
> > >
> >
> > The snapshot metadata (redirection table) is written *after* the copy.
> > Data will not hit the origin device until after the copy on write has
> > completed successfully.
> >
> > - Joe
> >
>
> --
>
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Heinz Mauelshagen Sistina Software Inc.
Senior Consultant/Developer Am Sonnenhang 11
56242 Marienrachdorf
Germany
Mauelshagen at Sistina.com +49 2626 141200
FAX 924446
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