On Thu, 12 Jul 2012, bofh wrote:

When I do a snapshot, that file is part of the snapshot.  But are
changes within the file kept as well?

Only the difference (at block level) between the snapshots is kept. If the changed data was changed 1000 times between the snapshots, then only the difference between the blocks at the exact time the snapshot was taken is preserved.

In other words, is it useful to have snapshots of a postgresql
database server?  If a database file uses 10% of the pool, and changes
20% between each snapshot, would the pool run out of space after 9
more snapshots?  Or would the pool continue to have 90% free space?
Or would only 70% or so free space?

If it changes 20% between each snapshot then you would indeed soon run out of space unless older snapshots are destroyed.

It is normally for only a tiny percentage of a filesystem's data to change on a day by day basis.

Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
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