On Wed, May 22, 2013 at 2:30 PM, Greg Smith <greg@xxxxxxxxxxxxxxx> wrote: > On 5/22/13 3:06 PM, Joshua D. Drake wrote: >> >> Greg, can you elaborate on the SSD + Xlog issue? What type of burn >> through are we talking about? > > > You're burning through flash cells at a multiple of the total WAL write > volume. The system I gave iostat snapshots from upthread (with the Intel > 710 hitting its limit) archives about 1TB of WAL each week. The actual > amount of WAL written in terms of erased flash blocks is even higher though, > because sometimes the flash is hit with partial page writes. The write > amplification of WAL is much worse than the main database. > > I gave a rough intro to this on the Intel drives at > http://blog.2ndquadrant.com/intel_ssds_lifetime_and_the_32/ and there's a > nice "Write endurance" table at > http://www.tomshardware.com/reviews/ssd-710-enterprise-x25-e,3038-2.html > > The cheapest of the Intel SSDs I have here only guarantees 15TB of total > write endurance. Eliminating >1TB of writes per week by moving the WAL off > SSD is a pretty significant change, even though the burn rate isn't a simple > linear thing--you won't burn the flash out in only 15 weeks. Certainly, intel 320 is not designed for 1tb/week workloads. > The production server is actually using the higher grade 710 drives that aim > for 900TB instead. But I do have standby servers using the low grade stuff, > so anything I can do to decrease SSD burn rate without dropping performance > is useful. And only the top tier of transaction rates will outrun a RAID1 > pair of 15K drives dedicated to WAL. s3700 is rated for 10 drive writes/day for 5 years. so, for 200gb drive, that's 200gb * 10/day * 365 days * 5, that's 3.65 million gigabytes or ~ 3.5 petabytes. 1tb/week would take 67 years to burn through / whatever you assume for write amplification / whatever extra penalty you give if you are shooting for > 5 year duty cycle (flash degrades faster the older it is) *for a single 200gb device*. write endurance is not a problem for this drive, in fact it's a very reasonable assumption that the faster worst case random performance is directly related to reduced write amplification. btw, cost/pb of this drive is less than half of the 710 (which IMO was obsolete the day the s3700 hit the street). merlin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
