2010/11/21 Andres Freund <andres@anarazel.de>:
> On Sunday 21 November 2010 23:19:30 Martijn van Oosterhout wrote:
>> For a similar problem we had (kernel buffering too much) we had success
>> using the fadvise and madvise WONTNEED syscalls to force the data to
>> exit the cache much sooner than it would otherwise. This was on Linux
>> and it had the side-effect that the data was deleted from the kernel
>> cache, which we wanted, but probably isn't appropriate here.
> Yep, works fine. Although it has the issue that the data will get read again if
> archiving/SR is enabled.
mmhh . the current code does call DONTNEED or WILLNEED for WAL
depending of the archiving off or on.
This matters *only* once the data is writen (fsync, fdatasync), before
that it should not have an effect.
>
>> There is also sync_file_range, but that's linux specific, although
>> close to what you want I think. It would allow you to work with blocks
>> smaller than 1GB.
> Unfortunately that puts the data under quite high write-out pressure inside
> the kernel - which is not what you actually want because it limits reordering
> and such significantly.
>
> It would be nicer if you could get a mix of both semantics (looking at it,
> depending on the approach that seems to be about a 10 line patch to the
> kernel). I.e. indicate that you want to write the pages soonish, but don't put
> it on the head of the writeout queue.
>
> Andres
>
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Cédric Villemain 2ndQuadrant
http://2ndQuadrant.fr/ PostgreSQL : Expertise, Formation et Support