Re: fallocate / posix_fallocate for new WAL file creation (etc...)

On Fri, May 17, 2013 at 4:47 AM, Andres Freund <andres(at)2ndquadrant(dot)com> wrote:
> On 2013-05-15 16:46:33 -0500, Jon Nelson wrote:
>> > * Is wal file creation performance actually relevant? Is the performance
>> > of a system running on fallocate()d wal files any different?
>>
>> In my limited testing, I noticed a drop of approx. 100ms per WAL file.
>> I do not have a good idea for how to really stress the WAL-file
>> creation area without calling pg_start_backup and pg_stop_backup over
>> and over (with archiving enabled).
>
> My point is that wal file creation usually isn't all that performance
> sensitive. Once the cluster has enough WAL files it will usually recycle
> them and thus never allocate new ones. So for this to be really
> beneficial it would be interesting to show different performance during
> normal running. You could also check out of how many extents a wal file
> is made out of with fallocate in comparison to the old style method
> (filefrag will give you that for most filesystems).

But why does it have to be *really* beneficial? We're already making
optional posix_fxxx calls and fallocate seems to do exactly what we
would want in this context. Even if the 100ms drop doesn't show up
all that often, I'd still take it just for the defragmentation
benefits and the patch is fairly tiny.

merlin