Re: [RFC] ext3: per-process soft-syncing data=ordered mode

[Al Boldi <a1426z@gawab.com>]

Jan Kara wrote:
> > Greetings!
> >
> > data=ordered mode has proven reliable over the years, and it does this
> > by ordering filedata flushes before metadata flushes.  But this
> > sometimes causes contention in the order of a 10x slowdown for certain
> > apps, either due to the misuse of fsync or due to inherent behaviour
> > like db's, as well as inherent starvation issues exposed by the
> > data=ordered mode.
> >
> > data=writeback mode alleviates data=order mode slowdowns, but only works
> > per-mount and is too dangerous to run as a default mode.
> >
> > This RFC proposes to introduce a tunable which allows to disable fsync
> > and changes ordered into writeback writeout on a per-process basis like
> > this:
> >
> >       echo 1 > /proc/`pidof process`/softsync
>
>   I guess disabling fsync() was already commented on enough. Regarding
> switching to writeback mode on per-process basis - not easily possible
> because sometimes data is not written out by the process which stored
> them (think of mmaped file).

Do you mean there is a locking problem?

> And in case of DB, they use direct-io
> anyway most of the time so they don't care about journaling mode anyway.

Testing with sqlite3 and mysql4 shows that performance drastically improves 
with writeback writeout.

>  But as Diego wrote, there is definitely some room for improvement in
> current data=ordered mode so the difference shouldn't be as big in the
> end.

Yes, it would be nice to get to the bottom of this starvation problem, but 
even then, the proposed tunable remains useful for misbehaving apps.


Thanks!

--
Al