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Re: [Commits] Rev 4376: MDEV-6676: Speculative parallel replication in http://bazaar.launchpad.net/~maria-captains/maria/10.0


Thanks Kristian. That's a nice optimization that is difficult to do outside
the DBMS engine.

Cheers, Robert

On Wed, Sep 10, 2014 at 2:39 PM, Kristian Nielsen <knielsen@xxxxxxxxxxxxxxx>

> Robert Hodges <robert.hodges@xxxxxxxxxxxxxx> writes:
> > There is one thing I have never understood about your parallel apply
> > algorithm.  How do you handle the case where the server crashes when some
> > threads have committed but others have not?  It seems as if you could
> have
> > a problem with recovery.
> Transactions are executed in parallel, but they are committed sequentially.
> So if we crash, there is always a single point before which all
> transactions
> are committed and after which no transactions are.
> If using InnoDB and global transaction ID, this point is saved in a
> crash-safe
> way, so no problems with crash recovery.
> If using MyISAM, or if using old filename/offset position in
> relay-log.info,
> then a crash may leave an inconsistency - but this is also the case with
> non-parallel replication.
> A substantial effort was spent in the code to make the sequential commit of
> the parallelly applied transaction efficient. For example, it is possible
> for
> such transactions to be group committed.
>  - Kristian.

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