New replication mode: write

Hi,

http://archives.postgresql.org/message-id/AANLkTilgyL3Y1jkDVHX02433COq7JLmqicsqmOsbuyA1%40mail.gmail.com

Previously I proposed the replication mode "recv" on the above thread,
but it's not
committed yet. Now I'd like to propose that mode again because it's
useful to reduce
the overhead of synchronous replication. Attached patch implements that mode.

If you choose that mode, transaction waits for its WAL to be write()'d
on the standby,
IOW, waits until the standby saves the WAL in the memory. Which provides lower
level of durability than that current synchronous replication (i.e.,
transaction waits for
its WAL to be flushed to the disk) does. However, it's practically
useful setting
because it can decrease the response time for the transaction, and
causes no data loss
unless both the master and the standby crashes and the database of the
master gets
corrupted at the same time.

In the patch, you can choose that mode by setting synchronous_commit to write.
I renamed that mode to "write" from "recv" on the basis of its actual behavior.

I measured how much "write" mode improves the performance in
synchronous replication.
Here is the result:

synchronous_commit = on
tps = 424.510843 (including connections establishing)
tps = 420.767883 (including connections establishing)
tps = 419.715658 (including connections establishing)
tps = 428.810001 (including connections establishing)
tps = 337.341445 (including connections establishing)

synchronous_commit = write
tps = 550.752712 (including connections establishing)
tps = 407.104036 (including connections establishing)
tps = 455.576190 (including connections establishing)
tps = 453.548672 (including connections establishing)
tps = 555.171325 (including connections establishing)

I used pgbench (scale factor = 100) as a benchmark and ran the
following command.

    pgbench -c 8 -j 8 -T 60 -M prepared

I always ran CHECKPOINT in both master and standby before starting each pgbench
test, to prevent CHECKPOINT from affecting the result of the performance test.

Thought? Comments?

Regards,

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center