New statistics for WAL buffer dirty writes

Hi all,

I've created new patch to get/reset statistics of WAL buffer
writes (flushes) caused by WAL buffer full.

This patch provides two new functions, pg_stat_get_xlog_dirty_write()
and pg_stat_reset_xlog_dirty_write(), which have been designed to
determine an appropriate value for WAL buffer size.

If this counter is increasing in the production environment,
it would mean that the WAL buffer size is too small to hold
xlog records generated the transactions. So, you can increase
your WAL buffer size to keep xlog records and to reduce WAL writes.

I think this patch would not affect to WAL write performance,
but still paying attention to it.

Any comments or suggestions?

Regards,

-----------------------------------------------------------
[snaga@devvm03 src]$ psql -p 15432 postgres
psql (9.3devel)
Type "help" for help.

postgres=# SELECT pg_stat_get_xlog_dirty_write();
 pg_stat_get_xlog_dirty_write
------------------------------
                            0
(1 row)

postgres=# \q
[snaga@devvm03 src]$ pgbench -p 15432 -s 10 -c 32 -t 1000 postgres
Scale option ignored, using pgbench_branches table count = 10
starting vacuum...end.
transaction type: TPC-B (sort of)
scaling factor: 10
query mode: simple
number of clients: 32
number of threads: 1
number of transactions per client: 1000
number of transactions actually processed: 32000/32000
tps = 141.937738 (including connections establishing)
tps = 142.123457 (excluding connections establishing)
[snaga@devvm03 src]$ psql -p 15432 postgres
psql (9.3devel)
Type "help" for help.

postgres=# SELECT pg_stat_get_xlog_dirty_write();
 pg_stat_get_xlog_dirty_write
------------------------------
                            0
(1 row)

postgres=# begin;
BEGIN
postgres=# DELETE FROM pgbench_accounts;
DELETE 1000000
postgres=# commit;
COMMIT
postgres=# SELECT pg_stat_get_xlog_dirty_write();
 pg_stat_get_xlog_dirty_write
------------------------------
                        19229
(1 row)

postgres=# SELECT pg_stat_reset_xlog_dirty_write();
 pg_stat_reset_xlog_dirty_write
--------------------------------

(1 row)

postgres=# SELECT pg_stat_get_xlog_dirty_write();
 pg_stat_get_xlog_dirty_write
------------------------------
                            0
(1 row)

postgres=# \q
[snaga@devvm03 src]$
-----------------------------------------------------------


--
Satoshi Nagayasu <snaga@uptime.jp>
Uptime Technologies, LLC. http://www.uptime.jp