Re: [Lsf-pc] Linux kernel impact on PostgreSQL performance

On Wed 15-01-14 10:12:38, Robert Haas wrote:
> On Wed, Jan 15, 2014 at 4:35 AM, Jan Kara <jack(at)suse(dot)cz> wrote:
> > Filesystems could in theory provide facility like atomic write (at least up
> > to a certain size say in MB range) but it's not so easy and when there are
> > no strong usecases fs people are reluctant to make their code more complex
> > unnecessarily. OTOH without widespread atomic write support I understand
> > application developers have similar stance. So it's kind of chicken and egg
> > problem. BTW, e.g. ext3/4 has quite a bit of the infrastructure in place
> > due to its data=journal mode so if someone on the PostgreSQL side wanted to
> > research on this, knitting some experimental ext4 patches should be doable.
>
> Atomic 8kB writes would improve performance for us quite a lot. Full
> page writes to WAL are very expensive. I don't remember what
> percentage of write-ahead log traffic that accounts for, but it's not
> small.
OK, and do you need atomic writes on per-IO basis or per-file is enough?
It basically boils down to - is all or most of IO to a file going to be
atomic or it's a smaller fraction?

As Dave notes, unless there is HW support (which is coming with newest
solid state drives), ext4/xfs will have to implement this by writing data
to a filesystem journal and after transaction commit checkpointing them to
a final location. Which is exactly what you do with your WAL logs so
it's not clear it will be a performance win. But it is easy enough to code
for ext4 that I'm willing to try...

Honza
--
Jan Kara <jack(at)suse(dot)cz>
SUSE Labs, CR