On Wed, Jun 4, 2014 at 9:21 AM, Tom Lane <tgl@xxxxxxxxxxxxx> wrote:
If your input data is uniformly incompressible, that's not too surprising.
pg_largeobject tuples hold BLCKSZ/4 bytes of data, plus some overhead, so
the only way that 4 of them will fit on a page is if compression saves
more than the overhead. You only need a couple percent compression
savings to make that work, but precompressed data might resist being
compressed even that much. So you end up with only 3 tuples per page
which makes it right about 33.33% overhead.
I have several filters compressing the data (shuffle, nbit, gzip). I could just disable gzip. Would this result in more efficient storage (ie less total GB used) beyond the per-block utilization bump? If that works, that would be counter intuitive and use more bandwidth (which I'm ok with) so it's a strange tradeoff.
According to the commentary in the source code (which you've evidently
read to some extent), a smaller block size was chosen to reduce the
overhead of partial updates of large-object data. You're probably not
going to do that, but PG doesn't know it. In any case, you can't choose
a LOBLKSIZE exceeding, or even equaling, the page size; so there's not
room for a huge change here. According to the above analysis, if you
want to pack more LO data per page, you'd actually be better off with
a *smaller* LOBLKSIZE to limit the per-page wastage.
I only looked at a header briefly :-). The rest was in the docs ML http://www.postgresql.org/docs/9.3/static/catalog-pg-largeobject.html
What would likely be the best bet for you is to run a modified build with
LOBLKSIZE held at 2K and BLCKSZ bumped up to 32K. That would cut the
wastage from 2K per 8K to 2K per 32K. Operationally, it'd be safer since
there *is* a pg_control cross-check on BLCKSZ, so you could not
accidentally start a standard build against the database.
Or you could just live with it. 10GB of disk is cheap insurance against
human error, and you're definitely taking some risk of human error if
you run a locally-patched build.
regards, tom lane
The overhead is noticeable as storage for me is a little hard to come by (budget limited R&D) but I respect that point. I think something should be added in the future though to match os file storage efficiency given the new lo limits are 4TB per entry - 30% is hefty overhead. Perhaps multiple large object tables such that the user can change the respective tables properties or whatever parameters needed being runtime specifiable to make this more efficient / higher utilization.
I may experiment with some custom builds in the future just to see how it goes.
-Jason
