Re: fallocate / posix_fallocate for new WAL file creation (etc...)

On 5/28/13 10:00 PM, Jon Nelson wrote:

> A note: The attached test program uses *fsync* instead of *fdatasync*
> after calling fallocate (or writing out 16MB of zeroes), per an
> earlier suggestion.

I tried this out on the RHEL5 platform I'm worried about now. There's
something weird about the test program there. If I run it once it shows
posix_fallocate running much faster:

without posix_fallocate: 1 open/close iterations, 1 rewrite in 23.0169s
with posix_fallocate: 1 open/close iterations, 1 rewrite in 11.1904s

The problem is that I'm seeing the gap between the two get smaller the
more iterations I run, which makes me wonder if the test is completely fair:

without posix_fallocate: 2 open/close iterations, 2 rewrite in 34.3281s
with posix_fallocate: 2 open/close iterations, 2 rewrite in 23.1798s

without posix_fallocate: 3 open/close iterations, 3 rewrite in 44.4791s
with posix_fallocate: 3 open/close iterations, 3 rewrite in 33.6102s

without posix_fallocate: 5 open/close iterations, 5 rewrite in 65.6244s
with posix_fallocate: 5 open/close iterations, 5 rewrite in 61.0991s

You didn't show any output from the latest program on your system, so
I'm not sure how it behaved for you here.

--
Greg Smith 2ndQuadrant US greg(at)2ndQuadrant(dot)com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com