Why does autovacuum run in so small blocks? - Mailing list pgsql-general

hi,
I have strange situation with one table.

base info: pg 8.4.8

here is info from pg_stat_all_tables about i:
              now              | relid | n_tup_ins | n_tup_upd | n_tup_del | n_tup_hot_upd | n_live_tup | n_dead_tup |
    last_autovacuum 

-------------------------------+-------+-----------+-----------+-----------+---------------+------------+------------+------------------------------
 2011-11-09 21:07:02.250232+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:07:22.312616+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:07:42.377629+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:08:02.491853+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:08:22.575746+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:08:42.641988+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:09:02.705319+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:09:22.769856+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:09:42.837779+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:10:02.903242+00 | xxx   |      4184 |   1069977 |         0 |        621580 |       4184 |          0 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:10:23.315874+00 | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |        507 |
2011-11-0921:06:35.23673+00 
 2011-11-09 21:10:43.38273+00  | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |        507 |
2011-11-0921:10:24.161642+00 
 2011-11-09 21:11:03.44763+00  | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |        507 |
2011-11-0921:10:24.161642+00 
 2011-11-09 21:11:23.51432+00  | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |          0 |
2011-11-0921:11:20.879516+00 
 2011-11-09 21:11:43.576631+00 | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |          0 |
2011-11-0921:11:20.879516+00 
 2011-11-09 21:12:03.643183+00 | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |          0 |
2011-11-0921:11:20.879516+00 
 2011-11-09 21:12:23.699986+00 | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |          0 |
2011-11-0921:11:20.879516+00 
 2011-11-09 21:12:43.762559+00 | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |          0 |
2011-11-0921:11:20.879516+00 
 2011-11-09 21:13:03.8274+00   | xxx   |      4184 |   1070484 |         0 |        621949 |       4184 |          0 |
2011-11-0921:11:20.879516+00 

as you can see within ~ 5 minutes, there were no inserts, just some updates.
Which explains (kind of) the autovacuum at 21:10:24.161642+00
but why was there another one at 21:11:20.879516+00?

In logs I see:

2011-11-09 21:06:35.236 GMT::@:[6957]:LOG:  automatic vacuum of table "public.xxx": index scans: 1
        pages: 0 removed, 1094 remain
        tuples: 704 removed, 2990 remain
        system usage: CPU 0.00s/0.01u sec elapsed 0.10 sec
--
2011-11-09 21:10:24.161 GMT::@:[8161]:LOG:  automatic vacuum of table "public.xxx": index scans: 0
        pages: 0 removed, 1094 remain
        tuples: 0 removed, 3516 remain
        system usage: CPU 0.00s/0.00u sec elapsed 0.05 sec
--
2011-11-09 21:11:20.879 GMT::@:[8467]:LOG:  automatic vacuum of table "public.xxx": index scans: 1
        pages: 0 removed, 1094 remain
        tuples: 507 removed, 2938 remain
        system usage: CPU 0.00s/0.01u sec elapsed 0.01 sec

the question basically is - why do we have so many vacuums?
especially the ones that don't do anything?

Best regards,

depesz

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