Re: Improvement of checkpoint IO scheduler for stable transaction responses

On Wed, Jul 3, 2013 at 4:18 AM, KONDO Mitsumasa
<kondo(dot)mitsumasa(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> I tested and changed segsize=0.25GB which is max partitioned table file size and
> default setting is 1GB in configure option (./configure --with-segsize=0.25).
> Because I thought that small segsize is good for fsync phase and background disk
> write in OS in checkpoint. I got significant improvements in DBT-2 result!

This is interesting. Unfortunately, it has a significant downside:
potentially, there will be a lot more files in the data directory. As
it is, the number of files that exist there today has caused
performance problems for some of our customers. I'm not sure off-hand
to what degree those problems have been related to overall inode
consumption vs. the number of files in the same directory.

If the problem is mainly with number of of files in the same
directory, we could consider revising our directory layout. Instead
of:

base/${DBOID}/${RELFILENODE}_{FORK}

We could have:

base/${DBOID}/${FORK}/${RELFILENODE}

That would move all the vm and fsm forks to separate directories,
which would cut down the number of files in the main-fork directory
significantly. That might be worth doing independently of the issue
you're raising here. For large clusters, you'd even want one more
level to keep the directories from getting too big:

base/${DBOID}/${FORK}/${X}/${RELFILENODE}

...where ${X} is two hex digits, maybe just the low 16 bits of the
relfilenode number. But this would be not as good for small clusters
where you'd end up with oodles of little-tiny directories, and I'm not
sure it'd be practical to smoothly fail over from one system to the
other.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company