Scott Marlowe wrote: > On Wed, Oct 20, 2010 at 8:25 PM, Joshua D. Drake <jd@xxxxxxxxxxxxxxxxx> 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. With a BBU you can turn off full_page_writes, which should decrease the WAL traffic. However, I don't see this mentioned in our documentation. Should I add it? -- Bruce Momjian <bruce@xxxxxxxxxx> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
