| From: | Manfred Spraul <manfred(at)colorfullife(dot)com> |
|---|---|
| To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
| Cc: | Doug McNaught <doug(at)mcnaught(dot)org>, Christopher Kings-Lynne <chriskl(at)familyhealth(dot)com(dot)au>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: O_DIRECT in freebsd |
| Date: | 2003-10-29 17:45:24 |
| Message-ID: | 3F9FFCB4.1020103@colorfullife.com |
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| Lists: | pgsql-hackers |
Tom Lane wrote:
> Not for WAL --- we never read the WAL at all in normal operation. (If
>
>it works for writes, then we would want to use it for writing WAL, but
>that's not apparent from what Christopher quoted.)
>
At least under Linux, it works for writes. Oracle uses O_DIRECT to
access (both read and write) disks that are shared between multiple
nodes in a cluster - their database kernel must know when the data is
visible to the other nodes.
One problem for WAL is that O_DIRECT would disable the write cache -
each operation would block until the data arrived on disk, and that
might block other backends that try to access WALWriteLock.
Perhaps a dedicated backend that does the writeback could fix that.
Has anyone tried to use posix_fadvise for the wal logs?
http://www.opengroup.org/onlinepubs/007904975/functions/posix_fadvise.html
Linux supports posix_fadvise, it seems to be part of xopen2k.
--
Manfred
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