On Thu, Jul 7, 2016 at 10:27 AM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: > On Wed, Jul 6, 2016 at 4:48 PM, Scott Marlowe <scott.marlowe@xxxxxxxxx> wrote: >> On Wed, Jul 6, 2016 at 12:13 PM, Merlin Moncure <mmoncure@xxxxxxxxx> wrote: >>> Disabling write back cache for write heavy database loads will will >>> destroy it in short order due to write amplication and will generally >>> cause it to underperform hard drives in my experience. >> >> Interesting. We found our best performance with a RAID-5 of 10 800GB >> SSDs (Intel 3500/3700 series) that we got MUCH faster performance with >> all write caching turned off on our LSI MEgaRAID controllers. We went >> from 3 to 4ktps to 15 to 18ktps. And after a year of hard use we still >> show ~90% life left (these machines handle thousands of writes per >> second in real use) It could be that the caching was getting in the >> way of RAID calcs or some other issue. With RAID-1 I have no clue what >> the performance will be with write cache on or off. > > Right -- by that I meant disabling the write back cache on the drive > itself, so that all writes are immediately flushed. Disabling write > back on the raid controller should be the right choice; each of these > drives essentially is a 'caching raid controller' for all intents and > purposes. Hardware raid controllers are engineered around performance > and reliability assumptions that are no longer correct in an SSD > world. Personally I would have plugged the drives directly to the > motherboard (assuming it's a got enough lanes) and mounted the raid > against mdadm and compared. Oh yeah definitely. And yea we've found that mdadm and raw HBAs work better than most RAID controllers for SSDs. -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
