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:
checkpointstarting: 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=0.637s  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=2.627s, average=0.563 s  LOG:  checkpoint starting: xlog  LOG: checkpoint complete: wrote 6148 buffers (37.5%);
0transaction 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=154.566s; 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.