Why autovacuum process takes so much memory and swap memory? - postgresql

Question

We have a product log database which only produces insert sql。 But we found that
The autovacuum process took so much memory。 it takes about 16 GB at peak, And it is
happend about every 2 month。The folloing is the detail information, Any one know it?
And the table skytf.urs_user_log_201105 only has insert operation, and have no
update,delete operations, So i think the table has no dead tuples! But why the autovacuum
process takes so much memory on the table?
--top detail
top - 16:39:46 up 225 days, 1:12, 1 user, load average: 1.29, 1.51, 1.52
Tasks: 341 total, 2 running, 339 sleeping, 0 stopped, 0 zombie
Cpu(s): 4.8%us, 5.3%sy, 0.0%ni, 85.5%id, 4.1%wa, 0.0%hi, 0.4%si, 0.0%st
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
29267 postgres 14 -1 27.0g 16g 2928 S 1.0 72.2 199:59.74 postgres: autovacuum launcher process
From the above, we can see that the autovacuum process taks about 16 gb;
--current sql
postgres=# select datname, current_query from pg_stat_activity where current_query !='<IDLE>';
datname | current_query
----------+-------------------------------------------------------------------------------------
skytf | autovacuum: VACUUM skytf.urs_user_log_201105 (to prevent wraparound)
--table size
skytf=> \dt+ urs_user_log_201105
List of relations
Schema | Name | Type | Owner | Size | Description
--------+---------------------+-------+--------+-------+-------------
skytf | urs_user_log_201105 | table | skytf | 62 GB |
(1 row)
--memory state
postgres#logdb-> free -m
total used free shared buffers cached
Mem: 24104 24028 75 0 4 5545
-/+ buffers/cache: 18479 5625
Swap: 16386 8824 7561

Answer 1

If you look at this:
autovacuum: VACUUM skytf.urs_user_log_201105 (to prevent wraparound)
this is not a regular autovacuum. it is running to prevent transaction wraparound. this kind of autovacuum must run before transaction id goes beyond two billion. read more at here:
http://www.postgresql.org/docs/8.3/static/routine-vacuuming.html#VACUUM-FOR-WRAPAROUND
you can control the behavior of it with vacuum_freeze_min_age, vacuum_freeze_max_age. you can not disable this kind of autovacuum whether you have disabled autovacuum or not.

Answer 2

only has insert operation, and have no update,delete operations, So i think the table has no dead tuples!
If memory serves, autovacuum will actually do two things:
vacuum
analyze
The first won't kick in if you only ever get inserts. But the second continues to do so when PG thinks the stats might have changed a bit.