Re: Snapshots and mmaped files.

"David Brown" <lvm@davidb.org> · Wed, 20 Apr 2005 21:06:36 -0700

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.

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