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.
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.
--
Greg Smith 2ndQuadrant US greg@2ndQuadrant.com Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support www.2ndQuadrant.com
