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Re: [linux-lvm] LVM Snapshots & journal recovery


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

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.
Second, to have these two extra waits seems really quite a bad penalty to

- 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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