Re: Wal sync odirect

On 07/21/2013 10:01 PM, Миша Тюрин wrote:
> hi, list. there are my proposal. i would like to tell about odirect in wal sync in wal_level is higher than minimal.
ithink in my case when wal traffic is up to 1gb per 2-3 minutes but discs hardware with 2gb bbu cache (or maybe ssd
underwal) - there would be better if wall traffic could not harm os memory eviction. and i do not use streaming. my
archivecommand may read wal directly without os cache. just opinion, i have not done any tests yet. but i am still
underthe some memory eviction anomaly.
 

PostgreSQL already uses O_DIRECT for WAL writes if you use O_SYNC mode
for WAL writes. See comments in src/include/access/xlogdefs.h (search
for O_DIRECT). You should also examine
src/backend/access/transam/xlog.c, particularly the function
get_sync_bit(...)

Try doing some tests with pg_test_fsync, see how performance looks. If
your theory is right and WAL traffic is putting pressure on kernel write
buffers, using fsync=open_datasync - which should be the default on
Linux - may help.

I'd recommend doing some detailed tracing and performance measurements
before trying to proceed further.

-- Craig Ringer                   http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services