Re: moving pg_xlog -- yeah, it's worth it!

Aidan Van Dyk <aidan@highrise.ca> wrote:
> Alvaro Herrera <alvherre@commandprompt.com> wrote:
>> Hmm, so maybe the performance benefit is not from it being on a
>> separate array, but from it being RAID1 instead of RAID5?
>
> Or the cumulative effects of:
> 1) Dedicated spindles/Raid1
> 2) More BBU cache available (I can't imagine the OS pair writing
>    much)
> 3) not being queued behind data writes before getting to
>    controller
> 3) Not waiting for BBU cache to be available (which is shared with
>    all data writes) which requires RAID5 writes to complete...
>
> Really, there's *lots* of variables here.  The basics being that
> WAL on the same FS as data, on a RAID5, even with BBU is worse
> than WAL on a dedicated set of RAID1 spindles with it's own BBU.
>
> Wow!

Sure, OK, but what surprised me was that a set of 15 read-only
queries (with pretty random reads) took almost twice as long when
the WAL files were on the same file system.  That's with OS writes
being only about 10% of reads, and *that's* with 128 GB of RAM which
keeps a lot of the reads from having to go to the disk.  I would not
have expected that a read-mostly environment like this would be that
sensitive to the WAL file placement.  (OK, I *did* request the
separate file system for them anyway, but I thought it was going to
be a marginal benefit, not something this big.)

-Kevin