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I have a server running apache, postgresql, and a lot of sites, and my graphs from my hosting provider indicate basically constant disk thrashing. 10k/s disk usage. This seems like a bad idea, but I'm not really sure why -- load is fine, server response time is reasonable. At first I thought that it was that the box was stuck in swap, because there were 2 application server threads for each site running, but reducing the number of sites running and the disk usage is still high. There's a little swap in use, but a lot of free memory. From top:

load average: 0.70, 0.28, 0.21

Cpu(s):  3.6%us,  1.4%sy,  0.0%ni, 93.9%id,  0.7%wa,  0.0%hi,  0.0%si,  0.4%st
Mem:   1731880k total,  1530124k used,   201756k free,    68360k buffers
Swap:  2359288k total,     1676k used,  2357612k free,   884892k cached

So I looked at vmstat, and it says swap isn't the issue:

root@foobar# vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0   1676 175220  68120 880856    0    0     1    20   20   15  4  1 94  1
 0  0   1676 174096  68140 881140    0    0     0  6484 1452  487  2  1 97  0
 0  0   1676 173848  68156 881420    0    0     0  5804  899  310  0  1 99  0
 0  0   1676 173600  68168 881768    0    0     0  5376  834  266  0  0 98  2
 0  0   1676 173228  68176 881880    0    0     0  5376  775  259  0  1 99  0
 1  0   1676 173044  68188 882152    0    0     0  3584  626  249  0  1 99  0
 0  0   1676 172796  68208 882424    0    0     0  6668 1450  593  2  1 97  0
 1  0   1676 181352  68224 882700    0    0     0  5760 2373 1038  3  2 95  0
 0  0   1676 181336  68228 882836    0    0     0  5376 3367 1289  7  1 91  0
 0  0   1676 181088  68236 883264    0    0     0  5376 1294  508  2  1 97  0
 0  0   1676 180848  68252 883440    0    0     0  5376 1217  410  1  1 98  0
 0  0   1676 180716  68264 883480    0    0     0  8444 1127  270  0  1 99  0
 0  0   1676 181584  68272 883488    0    0     0  5676 1285  624  1  1 98  0
 0  0   1676 180956  68272 883496    0    0     0  5376 1069  377  1  1 97  0
 0  0   1676 180964  68272 883496    0    0     0  5376  753  299  0  1 99  0

I tried increasing postgresql's shared_mem and work_mem settings, to no avail. I'm really not sure how to track down what's going on.

edit: here's iotop's big revelation:

26331 be/4 postgres    0.00 B/s    5.00 M/s  0.00 %  0.00 % postgres: stats collector process

running constantly, basically. Everything else is in the few kbps.

Any suggestions?

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  • Have you tried running iotop I feel that woud highlight the cause of the problem. Nov 5, 2012 at 19:30

1 Answer 1

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iotop will highlight the cause of the problem.

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  • it did. In this case, the problem was the postgres stats collector, and moving the stats collector to collect stats in an in-memory location reduced io usage to a reasonable level.
    – freyley
    Nov 5, 2012 at 20:34

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