Josh Berkus wrote:
On 4/8/09 9:44 AM, Tom Lane wrote:
Josh Berkus<j...@agliodbs.com> writes:
What about seq scans?
If the kernel can't read-ahead a seqscan by itself, it's unlikely to
be smart enough to be helped by posix_fadvise ... or at least so I
would think. Do you have reason to think differently?
Well, Solaris 10 + UFS should be helped by fadvise -- in theory at
least, it would eliminate the need to modify your mount points for
better readahead when setting up a PG-Solaris server. Solaris-UFS quite
lazy about readahead. Zdenek, Jignesh?
You're probably correct about Linux and FreeBSD. I don't know if OSX +
HFS supports fadvise. If so, it could only help; readahead on HFS right
now is nonexistant.
Presumably fadvise is useless on Windows. Anyone know?
It's important to distinguish what kind of fadvise we're talking about.
The bitmap scan code issues hints about individual pages, using
posix_fadvise(... POSIX_FADV_WILLNEED). For increasing the readahead of
a sequential scan, you'd want to use POSIX_FADV_SEQUENTIAL. I believe
the support for the latter is much more widespread than for the former.
xlog.c now also uses POSIX_FADV_WONTNEED to drop WAL pages from the OS
cache after writing them.
--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com
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