On Thu, Aug 15, 2013 at 12:08:57PM -0500, Jon Nelson wrote:
Where are we on this issue?
I've been able to replicate it pretty easily with PostgreSQL and
continue to look into it. I've contacted Theodore Ts'o and have gotten
some useful information, however I'm unable to replicate the behavior
with the test program (even one that's been modified). What I've
learned is:

- XLogWrite appears to take approx. 2.5 times longer when writing to a
file allocated with posix_fallocate, but only the first time the file
contents are overwritten. This is partially explained by how ext4
handles extents and uninitialized data, but 2.5x is MUCH more
expensive than anticipated or expected here.
- Writing zeroes to a file allocated with posix_fallocate (essentially
adding a posix_fallocate step before the usual write-zeroes-in-a-loop
approach) not only doesn't seem to hurt performance, it seems to help
or at least have parity, *and* the space is guaranteed to exist on
disk. At the very least that seems useful.
Is it time to revert this patch until we know more?

--
   Bruce Momjian <bruce@momjian.us> http://momjian.us
   EnterpriseDB http://enterprisedb.com

   + It's impossible for everything to be true. +

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