On Tue, Jun 11, 2013 at 11:08 AM, Stefan Drees <ste...@drees.name> wrote: > On 2013-11.06 17:28, Jon Nelson wrote: >> >> There hasn't been much activity here recently. I'm curious, then, if >> there are questions that I can answer. >> It may be useful to summarize some things here: >> >> - the purpose of the patch is to use posix_fallocate when creating new >> WAL files, because it's (usually) much quicker >> - using posix_fallocate is *one* system call versus 2048 calls to write(2) >> - additionally, using posix_fallocate /guarantees/ that the filesystem >> has space for the WAL file (by spec) >> - reportedly (difficult to test or prove), using posix_fallocate *may* >> reduce file fragmentation >> - the (limited) testing I've done bears this out: the more new WAL >> file creation there is, the more the improvement. Once the number of >> WAL files reaches a constant point, there does not appear to be either >> a positive or a negative performance impact. This is as expected. >> - a test program (C) was also written and used which creates, >> allocates, and then writes to files as fast as possible. This test >> program also shows the expected performance benefits. >> - the performance benefits range from a few percent up to about 15 percent > > > tried the test program of the patch at least a bit. > > Retrieved it from: > http://www.postgresql.org/message-id/attachment/29088/test_fallocate.c > > on an oldish linux box (Kernel 2.6.32, x86_64) following > $ gcc -o test_fallocate test_fallocate.c > a short > $ ./test_fallocate foo 1 1 > yields: > without posix_fallocate: 1 open/close iterations, 1 rewrite in 26.1993s > with posix_fallocate: 1 open/close iterations, 1 rewrite in 13.3299s > > on another box (Kernel 3.2.0, x86_64) same procedure yields: > without posix_fallocate: 1 open/close iterations, 1 rewrite in 19.1972s > with posix_fallocate: 1 open/close iterations, 1 rewrite in 9.9280s > > Note, when trying gcc -O2 test_fallocate.c fails to compile with: > > In file included from /usr/include/fcntl.h:252:0, > from test_fallocate.c:3: > In function ‘open’, > inlined from ‘main’ at test_fallocate.c:68:16: > /usr/include/x86_64-linux-gnu/bits/fcntl2.h:51:24: error: call to > ‘__open_missing_mode’ declared with attribute error: open with O_CREAT in > second argument needs 3 arguments
It's understood that posix_fallocate is faster at this -- the question on the table is 'does this matter in context of postgres?'. Personally I think this patch should go in regardless -- the concerns made IMNSHO are specious. merlin -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
