On Wed, Oct 20, 2010 at 8:25 PM, Joshua D. Drake <jd(at)commandprompt(dot)com> wrote:
> On Wed, 2010-10-20 at 22:13 -0400, Bruce Momjian wrote:
>> Ben Chobot wrote:
>> > On Oct 7, 2010, at 4:38 PM, Steve Crawford wrote:
>> >
>> > > I'm weighing options for a new server. In addition to PostgreSQL, this machine will handle some modest Samba and Rsync load.
>> > >
>> > > I will have enough RAM so the virtually all disk-read activity will be cached. The average PostgreSQL read activity will be modest - a mix of single-record and fairly large (reporting) result-sets. Writes will be modest as well but will come in brief (1-5 second) bursts of individual inserts. The rate of insert requests will hit 100-200/second for those brief bursts.
>> > >
>> > > So...
>> > >
>> > > Am I likely to be better off putting $$$ toward battery-backup on the RAID or toward adding a second RAID-set and splitting off the WAL traffic? Or something else?
>> >
>> > A BBU is, what, $100 or so? Adding one seems a no-brainer to me.
>> > Dedicated WAL spindles are nice and all, but they're still spinning
>> > media. Raid card cache is waaaay faster, and while it's best at bursty
>> > writes, it sounds like bursty writes are precisely what you have.
>>
>> Totally agree!
>
> BBU first, more spindles second.
Agreed. note that while you can get incredible burst performance from
a battery backed cache, due to both caching and writing out of order,
once the throughput begins to saturate at the speed of the disk array,
the bbu cache is now only re-ordering really, as it will eventually
fill up faster than the disks can take the writes, and you'll settle
in at some percentage of your max tps you get for a short benchmark
run. It's vitally important that once you put a BBU cache in place,
you run a very long running transactional test (pgbench is a simple
one to start with) that floods the io subsystem so you see what you're
average throughput is with the WAL and data store getting flooded. I
know on my system pgbench runs of a few minutes can be 3 or 4 times
faster than runs that last for the better part of an hour.