On Oct 10, 2010, at 10:14 AM, Mladen Gogala wrote: > > SQL> show parameter db_file_multi > > NAME TYPE VALUE > ------------------------------------ ----------- > ------------------------------ > db_file_multiblock_read_count integer 16 > SQL> alter session set db_file_multiblock_read_count=1; > > Session altered. > SQL> select count(*) from ni_occurrence; > > COUNT(*) > ---------- > 402062638 > > Elapsed: 00:08:20.88 > SQL> alter session set db_file_multiblock_read_count=128; > > Session altered. > > Elapsed: 00:00:00.50 > SQL> select count(*) from ni_occurrence; > > COUNT(*) > ---------- > 402062638 > > Elapsed: 00:02:17.58 > > > In other words, when I batched the sequential scan to do 128 blocks I/O, > it was 4 times faster than when I did the single block I/O. > Does that provide enough of an evidence and, if not, why not? > Did you tune the linux FS read-ahead first? You can get large gains by doing that if you are on ext3. blockdev --setra 2048 <device> would give you a 1MB read-ahead. Also, consider XFS and its built-in defragmentation. I have found that a longer lived postgres DB will get extreme file fragmentation over time and sequential scans end up mostly random. On-line file defrag helps tremendously. > It maybe so, but slow sequential scan is still the largest single > performance problem of PostgreSQL. The frequency with which that topic > appears on the mailing lists should serve as a good evidence for that. I > did my best to prove my case. I'm not sure its all the I/O however. It seems that Postgres uses a lot more CPU than other DB's to crack open a tuple and inspect it. Testing on unindexed tables with count(*) I can get between 200MB and 800MB per second off disk max with full cpu utilization (depending on the average tuple size and contents). This is on a disk array that can do 1200MB/sec. It always feels dissapointing to not be able to max out the disk throughput on the simplest possible query. > Again, requiring "hard numbers" when > using the database which doesn't allow tweaking of the I/O size is self > defeating proposition. The other databases, like DB2 and Oracle both > allow tweaking of that aspect of its operation, Oracle even on the per > session basis. If you still claim that it wouldn't make the difference, > the onus to prove it is on you. > > -- > Mladen Gogala > Sr. Oracle DBA > 1500 Broadway > New York, NY 10036 > (212) 329-5251 > www.vmsinfo.com > > > -- > Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
