Re: postgresql latency & bgwriter not doing its job

> What are the other settings here? checkpoint_segments,
> checkpoint_timeout, wal_buffers?

They simply are the defaults:

checkpoint_segments = 3
checkpoint_timeout = 5min
wal_buffers = -1

I did some test checkpoint_segments = 1, the problem is just more frequent
but shorter. I also reduced wal_segsize down to 1MB, which also made it
even more frequent but much shorter, so the overall result was an
improvement with 5% to 3% of transactions lost instead of 10-14%, if I
recall correctly. I have found no solution on this path.

> Could you show the output of log_checkpoints during that run? Checkpoint
> spreading only works halfway efficiently if all checkpoints are
> triggered by "time" and not by "xlog".

I do 500 seconds tests, so there could be at most 2 timeout triggered
checkpoints. Given the write load it takes about 2 minutes to fill the 3
16 MB buffers (8 kb * 50 tps (there is one page modified per transaction)
* 120 s ~ 48 MB), so checkpoints are triggered by xlog. The maths are
consistent with logs (not sure which prooves which, though:-):

LOG: received SIGHUP, reloading configuration files
LOG: parameter "log_checkpoints" changed to "on"
LOG: checkpoint starting: xlog
LOG: checkpoint complete: wrote 5713 buffers (34.9%); 0 transaction log
file(s) added, 0 removed, 0 recycled; write=51.449 s, sync=4.857 s,
total=56.485 s; sync files=12, longest=2.160 s, average=0.404 s
LOG: checkpoint starting: xlog
LOG: checkpoint complete: wrote 6235 buffers (38.1%); 0 transaction log
file(s) added, 0 removed, 3 recycled; write=53.500 s, sync=5.102 s,
total=58.670 s; sync files=8, longest=2.689 s, average=0.637 s
LOG: checkpoint starting: xlog
LOG: checkpoint complete: wrote 6250 buffers (38.1%); 0 transaction log
file(s) added, 0 removed, 3 recycled; write=53.888 s, sync=4.504 s,
total=58.495 s; sync files=8, longest=2.627 s, average=0.563 s
LOG: checkpoint starting: xlog
LOG: checkpoint complete: wrote 6148 buffers (37.5%); 0 transaction log
file(s) added, 0 removed, 3 recycled; write=53.313 s, sync=6.437 s,
total=59.834 s; sync files=8, longest=3.680 s, average=0.804 s
LOG: checkpoint starting: xlog
LOG: checkpoint complete: wrote 6240 buffers (38.1%); 0 transaction log
file(s) added, 0 removed, 3 recycled; write=149.008 s, sync=5.448 s,
total=154.566 s; sync files=9, longest=3.788 s, average=0.605 s

Note that my current effective solution is to do as if
"checkpoints_timeout = 0.2s": it works fine if I do my own spreading.

--
Fabien.