FAQ

On 6/19/13 11:02 AM, Claudio Freire wrote:
On Wed, Jun 19, 2013 at 7:13 AM, Tatsuo Ishii wrote:

For now, my idea is pretty vague.

- Record info about modified blocks. We don't need to remember the
whole history of a block if the block was modified multiple times.
We just remember that the block was modified since the last
incremental backup was taken.

- The info could be obtained by trapping calls to mdwrite() etc. We need
to be careful to avoid such blocks used in xlogs and temporary
tables to not waste resource.

- If many blocks were modified in a file, we may be able to condense
the info as "the whole file was modified" to reduce the amount of
info.

- How to take a consistent incremental backup is an issue. I can't
think of a clean way other than "locking whole cluster", which is
obviously unacceptable. Maybe we should give up "hot backup"?

I don't see how this is better than snapshotting at the filesystem
level. I have no experience with TB scale databases (I've been limited
to only hundreds of GB), but from my limited mid-size db experience,
filesystem snapshotting is pretty much the same thing you propose
there (xfs_freeze), and it works pretty well. There's even automated
tools to do that, like bacula, and they can handle incremental
snapshots.
A snapshot is not the same as an incremental backup; it presents itself as a full copy of the filesystem. Actually, since it's on the same underlying storage a snapshot isn't really a good backup at all.

The proposal (at least as I read it) is to provide a means to easily deal with *only* the data that has actually *changed* since the last backup was taken.
--
Jim C. Nasby, Data Architect jim@nasby.net
512.569.9461 (cell) http://jim.nasby.net

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postedJun 19, '13 at 10:14a
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