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RE: [linux-lvm] Merging snapshots - does lvmerge or similar exist forLinux



Following up the mail below, some more "Googling" turned up this patch...

http://www.gnome.org/~markmc/code/lvm-snapshot-merging/lvm-patches/lvm-lvmer
ge.patch

which seems fairly recent (2006-05-10).

Has anyone tried it or know anything about it?

> -----Original Message-----
> From: linux-lvm-bounces redhat com [mailto:linux-lvm-bounces redhat com]
> On Behalf Of Roger Lucas
> Sent: 17 August 2006 14:00
> To: 'LVM general discussion and development'
> Subject: [linux-lvm] Merging snapshots - does lvmerge or similar exist
> forLinux
> 
> Hi,
> 
> I have seen a few questions about this but could not find any answers.
> 
> I am working with Debian, the 2.16.20 kernel and the latest LVM+DMAPPER
> releases.
> 
> I have a logical volume that I want to be able to make changes to, but
> unroll these changes again if necessary.  When the operations are
> finished,
> I want to end up with just one LV and no additional snapshots.  The LV
> would
> either have its original contents or the updated contents, and this
> decision
> isn't made until all the changes have been done.
> 
> With LVM2, the snapshots are r/w so I could either:
> 
> 1) Snapshot the original LV and keep the snapshot safe, then make the
> changes to the original LV.  If I don't need to restore the LV after
> applying the changes, then I can just delete the snapshot.  If I do want
> to
> restore the original LV, however, what is the best way to flush the
> changes
> in the snapshot back onto the original LV ?
> 
> I appreciate that I could just "dd" the snapshot onto the LV to restore
> its
> orignal state, but this will rewrite a lot of data that hasn't actually
> changed.  Apart from being slower than I would like, I don't know if
> writing
> exactly the same data to the LV will trigger the data to be written to the
> snapshot - if it does then the snapshot will grow in size to the original
> LV
> whilst the "dd" happens, and I do not have enough disk space for that.
> 
> Or
> 
> 2) Snapshot the original LV and make the changes to the snapshot.  If I
> want
> to restore the original LV, then all I need to do is delete the snapshot.
> But, if I want to keep the changes (and not have to keep the snapshot),
> how
> can I flush the changes that have been made to the snapshot back onto the
> original LV.
> 
> I appreciate that I could just "dd" the snapshot onto the LV to update it
> to
> the new values, but I don't like this for the same reasons as in (1)
> above.
> 
> Is there an "lvmerge" command or similar on Linux LVM.  There appears to
> be
> one for the HP-UX LVM but I couldn't see one for Linux.
> 
> Alternatively, is the snapshot disk format easy enough to analyse so that
> a
> lvmerge utility could be written relatively easily ?
> 
> Thanks,
> 
> Roger
> 
> 
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