| From: | Florian Weimer <fw(at)deneb(dot)enyo(dot)de> |
|---|---|
| To: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Merlin Moncure <mmoncure(at)gmail(dot)com>, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>, Joachim Wieland <joe(at)mcknight(dot)de>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: posix_fadvise missing in the walsender |
| Date: | 2013-03-01 21:51:13 |
| Message-ID: | 871ubyaiwe.fsf@mid.deneb.enyo.de |
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| Lists: | pgsql-hackers |
* Jeff Janes:
> Does the kernel really read a data block from disk into memory in
> order to immediately overwrite it? I would have thought it would
> optimize that away, at least if the writes are sized and aligned to
> 512 or 1024 bytes blocks (which WAL should be).
With Linux, you'd have to use O_DIRECT to get that effect (but don't
do that), otherwise writes happen in page size granularity, writing in
512 or 1024 byte blocks should really trigger a read-modify-write
cycle.
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