Re: pgbench vs. wait events

On Thu, Oct 6, 2016 at 4:40 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> Scale factor 3000 obviously doesn't fit in shared_buffers. But does it fit
> in RAM? That is, are the backends doing real IO, or they just doing fake IO
> to the kernel's fs cache?

That's a good question.

[rhaas(at)hydra ~]$ free -g
total used free shared buffers cached
Mem: 61 26 34 0 0 24
-/+ buffers/cache: 2 58
Swap: 19 4 15

rhaas=# select pg_size_pretty(pg_relation_size('pgbench_accounts'));
pg_size_pretty
----------------
38 GB
(1 row)

rhaas=# select pg_size_pretty(pg_database_size(current_database()));
pg_size_pretty
----------------
44 GB
(1 row)

That's pretty tight, especially since I now notice Andres also left a
postmaster running on this machine back in April, with
shared_buffers=8GB. 44GB for this database plus 8GB for
shared_buffers plus 8GB for the other postmaster's shared_buffers
leaves basically no slack, so it was probably doing quite a bit of
real I/O, especially after the database got a bit of bloat.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company