Обсуждение: insert speed - Mac OSX vs Redhat
I've read most of the threads on insert speed in this list and wanted to share some interesting observations and a question. We've been benchmarking some dbs to implement Bayesian processing on an email server. This involves frequent insert and updates to the following table: create table bayes_token ( username varchar(200) not null default '', token varchar(200) not null default '', spam_count integer not null default 0, ham_count integer not null default 0, atime integer not null default 0, primary key (username, token)); On a variety of hardware with Redhat, and versions of postgres, we're not getting much better than 50 inserts per second. This is prior to moving WAL to another disk, and fsync is on. However, with postgres 7.4 on Mac OSX 10.2.3, we're getting an amazing 500 inserts per second. We can only put this down to the OS. Can anyone shed light on why Redhat appears to be so much poorer than Mac OS X in supporting postgres insert transactions? Or why MacOS appears to be so much better? BTW, on the same hardware that postgres is running on to get 50 inserts per sec, MySQL (4.0.17) is getting an almost unbelievable 5,500 inserts per second. -SL
> On a variety of hardware with Redhat, and versions of postgres, we're > not getting much better than 50 inserts per second. This is prior to > moving WAL to another disk, and fsync is on. > > However, with postgres 7.4 on Mac OSX 10.2.3, we're getting an amazing > 500 inserts per second. > > We can only put this down to the OS. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ You haven't really produced much evidence to support that statement. Given that the differences in performance between Postgres running on *BSD and Linux on Intel hardware are not large at all, it seems to be almost certainly false in fact. It may of course be due to some settings of the different OSes, but not the OSes themselves. It would help if you gave a straight PG7.4 comparison with hardware specs as well, and config file differences if any. One thought: assuming the Apple has IDE disks, then the disks probably have write caching turned on, which is good for speed,but not crash-safe. matt
Syd <569170@aliencamel.com> writes:
> However, with postgres 7.4 on Mac OSX 10.2.3, we're getting an amazing
> 500 inserts per second.
> We can only put this down to the OS.
As noted elsewhere, it's highly likely that this has nothing to do with
the OS, and everything to do with write caching in the disks being used.
I assume you are benchmarking small individual transactions (one insert
per xact). In such scenarios it's essentially impossible to commit more
than one transaction per revolution of the WAL disk, because you have to
write the same WAL disk page repeatedly and wait for it to get down to
the platter. When you get results that are markedly in excess of the
disk RPM figure, it's proof positive that the disk is lying about write
complete (or that you don't have fsync on).
The only way to get better performance and still have genuine ACID
behavior is to gang multiple insertions per WAL write. You can do
multiple insertions per transaction, or if you are doing several
insertion transactions in parallel, you can try to commit them all in
one write (experiment with the commit_delay and commit_siblings
parameters).
> BTW, on the same hardware that postgres is running on to get 50 inserts
> per sec, MySQL (4.0.17) is getting an almost unbelievable 5,500 inserts
> per second.
I'll bet a good lunch that MySQL is not being ACID compliant in this
test. Are you using a transaction-safe table type (InnoDB) and
committing after every insert?
If you don't in fact care about ACID safety, turn off fsync in Postgres
so that you have an apples-to-apples comparison (or at least
apples-to-oranges rather than apples-to-cannonballs).
regards, tom lane
On 16/01/2004, at 2:44 AM, Tom Lane wrote: ... > As noted elsewhere, it's highly likely that this has nothing to do with > the OS, and everything to do with write caching in the disks being > used. > > I assume you are benchmarking small individual transactions (one insert > per xact). In such scenarios it's essentially impossible to commit > more > than one transaction per revolution of the WAL disk, because you have > to > write the same WAL disk page repeatedly and wait for it to get down to > the platter. When you get results that are markedly in excess of the > disk RPM figure, it's proof positive that the disk is lying about write > complete (or that you don't have fsync on). > Tom, thanks for this explanation - we'll check this out straight away, but it would explain a lot.
We've found these tools http://scsirastools.sourceforge.net/ and http://www.seagate.com/support/seatools/ (for seagate drives) to check the settings of scsi disks and to change settings for seagate drives. What are people using for IDE disks? Are you all using hdparm on linux http://freshmeat.net/projects/hdparm/?topic_id=146%2C861 or are there other tools?