Postgresql update op is very very slow

I've a table with about 34601755 rows ,when I execute 'update msg_table set
type=0;' is very very slow, cost several hours, but still not complete?

Why postgresql is so slowly? Is the PG MVCC problem?

But I try it on Mysql, the same table and rows, it only cost about 340
seconds.

Any idea for the problem?

My machine config:
Memory 8G, 8 piece 15K disk , 2CPU(Quad-Core) AMD
OS: Red Hat AS4

My postgres.conf main parameter is following:

shared_buffers = 5GB # min 128kB or max_connections*16kB
# (change requires restart)
temp_buffers = 512MB # min 800kB
work_mem = 400MB # min 64kB
maintenance_work_mem = 600MB # min 1MB
max_fsm_pages = 262144 # 2G min max_fsm_relations*16, 6 bytes each
max_fsm_relations = 2000 # min 100, ~70 bytes each

bgwriter_delay = 20ms # 10-10000ms between rounds
bgwriter_lru_maxpages = 500 # 0-1000 max buffers written/round
bgwriter_lru_multiplier = 2.0 # 0-10.0 multipler on buffers