[linux-lvm] Snapshots and mmaped files.

[Dan Stromberg]

If lvm2 snapshots are like most snapshots, then they aren't a panacea -
they only decrease the window during which your database must be
quiescent.

EG, with snapshotting:

1) Shut down database
2) bifurcate off a snapshot
3) Start up database
4) Do backup against snapshot
5) Eliminate snapshot


EG without snapshotting:

1) Shut down database
2) Do backup
3) Start up database


The first case has more steps, but it also has a lot shorter time during
which your database is inactive.

On Tue, 2005-04-19 at 10:08 -0700, David Brown wrote:
> I'm using lvm2 snapshots to do backups of several machines.  (lvm2  
> 2.00.33, device-mapper 1.01.00,
> ftp://sources.redhat.com/pub/dm/patches/2.6-unstable/2.6.9/2.6.9-udm1.tar.bz2  
> minus patch 13)  (I've tried some newer patches, without success, but  
> that'll be a separate mail).
> 
> The problem shows up with files that contains Berkeley databases.  These  
> files seem to be modified on both the original and the snapshot.  They  
> show up in my 'var' partition with imapd, and exim, and anything else that  
> has a berkeley database opened, and modifies the file after the snapshot  
> is taken.
> 
> The mtime of the file is not modified, only the contents.  I suspect that  
> if one side grew, then the size change would not be affected.
> 
> Thanks,
> David Brown
> 
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> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
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