Re: fix use of posix_fadvise in xlog.c

Tom Lane wrote:
> Heikki Linnakangas <heikki.linnakangas@enterprisedb.com> writes:
>   
>> In a steady-state situation new WAL files are not created very often 
>> because we recycle old ones, so it probably doesn't make much difference.
>>     
>
> Yeah.  We really don't worry too much about the performance of the
> new-WAL-file-creation code path because of this.
>   

The only situation where the WAL zeroing path turns ugly is if you 
launch a bunch of activity against a fresh server that doesn't have any 
segments to recycle yet.  The last time we talked about improving that, 
the best idea I thought came out was to be better about preallocating 
segments than the code already is, rather than trying to speed up how 
the kernel deals with the situation.  See the links for "Be more 
aggressive about creating WAL files" at http://wiki.postgresql.org/wiki/Todo

I'm also not very optimistic about adding more posix_fadvise calls 
really helping just because the implementations of those are so 
unpredictable across operating systems.  I'm sure that Mark could figure 
out the right magic to speed up this specific case on Linux, but have my 
doubts that work would translate very well to many other operating 
systems.  Whereas a more generic preallocation improvement would help 
everywhere.

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us