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From Stefan Seelmann <seelm...@apache.org>
Subject Re: Backend recovery
Date Thu, 20 Nov 2008 23:51:13 GMT
Hi Emmanuel

Emmanuel Lecharny wrote:
> Hi guys,
> 
> as I'm currently working on a recovery tool, I have reviewed the current
> configuration we have. Let's first analyse how a recovery tool is good
> for and how it should work.
> 
> 1) Usage
> ...
Makes sense.

> 2) How it should work
> ...
> (a), or, worse, it occurs while we are writing those modifications, and
> now, we can have a totally unstable base (b).
> ...
> Case (b) is more problematic, because we have no way to determinate
> which was the previous stable state (Si), and to restore this state, as
> the base has already been partially modified.
Does JDBM provide tools to recover the latest stable state?

> 
> What is the solution ? We have to assume that the state Si can be
> restored, and that we can apply every modification on it. The only way
> to do that is to combine two techniques :
> - backup the base on regular basis, assuming that it can be done without
> allowing any update on this base during the backup (and this is not
> obvious, as the base can be pretty big)
> - and store every modification into a journal, to be able to replay them
> on the restored base.
Do I understand right that you want to write and sync the journal (the
changelog LDIF) immediately but the partition only from time to time? I
think that makes only sense if writing the journal is cheaper and faster
than writing the partition data.

Hm, that is similar to journaling file systems, right?

> 
> the N-1 backup). Here is the algorithm :
> 
> Journal last position is N, we have n modifications since then
> Spare current base is version N
> 
> - time to apply the journal ! Mark the current position in the journal
> as N+1
> - copy the spare base (Spare-N) to Spare-(N+1)
> - apply the journal from position N to position N+1
> - if everything is ok, tell the system that the current backup base is
> Spare-(N+1) and that the current log position is N+1
Ideally this should be an atomic operation.

Sound good.

Regards,
Stefan


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