maria-developers team mailing list archive

[Robert Hodges <robert.hodges@xxxxxxxxxxxxxx>]

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>
wrote:

> 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.
>