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List:       postgresql-general
Subject:    Re: [HACKERS] fallocate / posix_fallocate for new WAL file creation (etc...)
From:       Greg Smith <greg () 2ndQuadrant ! com>
Date:       2013-06-30 23:49:20
Message-ID: 51D0C400.1080001 () 2ndQuadrant ! com
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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@2ndQuadrant.com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com


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