PostgreSQL. Slow Prepare Transaction and Commit Prepared

StackOverflow https://stackoverflow.com//questions/12705412

Question

I've just encountered a strange problem. I've made a report in pgfourine, and found out that my XA transactions started to work really slow. Prepare transaction and commit prepared combined took 12.55s out of 13.2s. But why?

#####  Overall statistics  #####

Number of unique normalized queries: 175
Number of queries:     268,772
Total query duration:  13m2s


#####  Queries by type  #####

SELECT:    116493     43.3%
INSERT:     15926      5.9%
UPDATE:      7935      3.0%
DELETE:      4923      1.8%


#####  Queries that took up the most time (N)  #####

1) 6m32s - 26,338 - COMMIT PREPARED ''
--
2) 6m23s - 25,972 - PREPARE TRANSACTION ''
--
3) 0.6s - 3,848 - update avatar set lfa_position=NULL where Id=0
.....
7) 0.3s - 21,514 - COMMIT
.....

I have a theory but don't have a proof.. I have slow discs and I turned off synchronous_commit. Maybe PostgreSQL has to make an fsync during "prepare transaction" even if synchronous_commit is off? 

fsync = on 
synchronous_commit = off

Any ideas?

UPDATE

Same tests with 

fsync = off
synchronous_commit = off


#####  Overall statistics  #####

Number of unique normalized queries: 155
Number of queries:     186,838
Total query duration:  6.6s


#####  Queries by type  #####

SELECT:     84367     45.2%
INSERT:      9197      4.9%
UPDATE:      5486      2.9%
DELETE:      2996      1.6%


#####  Queries that took up the most time (N)  #####

1) 1.8s - 16,972 - PREPARE TRANSACTION ''
--
2) 1.1s - 16,965 - COMMIT PREPARED ''
--
3) 0.4s - 2,904 - update avatar set lfa_position=NULL where Id=0
--
4) 0.2s - 16,031 - COMMIT

Looks like fsync took vast amount of time, but not all the time. 16k commits - 0.2sec, 17k prepare+commit 2.9sec.

Sad story. Looks like XA commit tooks 15 times more time than local commit and doesn't take in account synchronous_commit setting. fsync=off is not safe for production use. So if I want to use XA transactions I have to use it carefully and use a good SSD drive with high IOPS.

Was it helpful?

Solution

The theory that PREPARE TRANSACTION is immediately sync'ed is correct, and it's mentioned in the doc:

Excerpt from http://www.postgresql.org/docs/9.1/static/wal-async-commit.html :

The commands supporting two-phase commit, such as PREPARE TRANSACTION, are also always synchronous.

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