[linux-lvm] Snapshots and mmaped files.

[linux-lvm] Snapshots and mmaped files.

[David Brown]

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

Re: [linux-lvm] Snapshots and mmaped files.

[Dan Stromberg]

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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
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

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Re: [linux-lvm] Snapshots and mmaped files.

[David Brown]

I don't think this is an issue of the database needing to be quiescent.   
The problem I'm seeing is that when the application modifies the database  
_after_ the snapshot is taken, the modifications partially appear in the  
snapshot as well.  I'm guessing that file modification that goes through  
write does the right thing in the snapshot, but modifications done via  
mmap don't cause the snapshot to duplicate the data.

The Berkely DB code should be able to recover from a coherent snapshot  
taken at any point in time.

Dave

On Wed, 20 Apr 2005 14:36:19 -0700, Dan Stromberg  
<strombrg@dcs.nac.uci.edu> wrote:

> 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.