pgpgout/s without swapping--what does it mean? - Mailing list pgsql-general

We had a big increase in load, iowait, and disk i/o on a dedicated database host the other night. 

Looking at the sar logs, the problem shows itself in a big increase in pgpgout/s, which I believe is postgres paging out parts of itself to disk?

02:15:01 AM  pgpgin/s pgpgout/s   fault/s  majflt/s  pgfree/s pgscank/s pgscand/s pgsteal/s    %vmeff

02:25:01 AM     49.63    163.83    836.80      0.00    467.69      4.80      0.00      4.80    100.00

02:35:01 AM     44.85    230.29   1248.68      0.00    677.18     16.21      0.00     16.21    100.00

02:45:01 AM     47.67    190.82   1059.58      0.00    517.72      5.33      0.00      5.33    100.00

02:55:01 AM     59.00    175.58    986.87      0.00    514.08     18.13      0.00     18.13    100.00

03:05:01 AM     24.67    849.77   1382.60      0.00   1267.94     37.35      0.00     37.35    100.00

03:15:01 AM     28.67   1701.67    717.88      0.00   1231.48      0.00      0.00      0.00      0.00

03:25:02 AM     42.64  21342.02   4086.19      0.04   9701.70    415.92      0.00    414.44     99.64

03:35:01 AM     35.60  28290.69   4305.38      0.10  12906.73    623.89      0.00    615.85     98.71

03:45:01 AM     36.94  31119.30   3675.34      0.01  12456.54    527.55      0.00    521.61     98.87

03:55:01 AM     42.77  29020.72   3458.96      0.01  12165.57    557.57      0.00    553.10     99.20

Average:        41.25  11306.39   2175.59      0.02   5189.70    220.62      0.00    218.63     99.10

However, there isn't a corresponding increase in pages *in*, so if postgres is writing portions of itself out to disk, they can't be very important.  And there's no swapping going on:

02:15:01 AM  pswpin/s pswpout/s

02:25:01 AM      0.00      0.00

02:35:01 AM      0.00      0.00

02:45:01 AM      0.00      0.00

02:55:01 AM      0.00      0.00

03:05:01 AM      0.00      0.00

03:15:01 AM      0.00      0.00

03:25:02 AM      0.00      0.00

03:35:01 AM      0.00      0.00

03:45:01 AM      0.00      0.00

03:55:01 AM      0.00      0.00

   Average:         0.00      0.00

Can anybody help me understand what these statistics are suggesting, what's actually going on on this box/in postgresql?  What is it writing to disk, and why?  Is it just writing out new/changed rows, or what?