Re: Performance PG 8.0 on dual opteron / 4GB / 3ware

Joost,

On 11/15/05 8:35 AM, "Joost Kraaijeveld" <J(dot)Kraaijeveld(at)Askesis(dot)nl> wrote:

> thousand go relatively fast, after that PostgreSQL crawls to a halt
> (other "benchmarks" like bonnie++ or just dd'ing a big file don't have
> this behavior).

With RAID5, it could matter a lot what block size you run your ³dd bigfile²
test with. You should run ³dd if=/dev/zero of=bigfile bs=8k count=500000²
for a 2GB main memory machine, multiply the count by (<your mem>/2GB).

It is very important with the 3Ware cards to match the driver to the
firmware revision.

> I did notice that changing the I/O scheduler's nr_request from the
> default 128 to 1024 or even 4096 made a remarkable performance
> improvement. I suspect that experimenting with other I/O schedululers
> could improve performance. But it is hard to find any useful
> documentation about I/O schedulers.
>
You could try deadline, there¹s no harm, but I¹ve found that when you reach
the point of experimenting with schedulers, you are probably not addressing
the real problem.
>
On a 3Ware 9500 with HW RAID5 and 4 or more disks I think you should get
100MB/s write rate, which is double what Postgres can use. We find that
Postgres, even with fsync=false, will only run at a net COPY speed of about
8-12 MB/s, where 12 is the Bizgres number. 8.1 might do 10. But to get the
10 or 12, the WAL writing and other writing is about 4-5X more than the net
write speed, or the speed at which the input file is parsed and read into
the database.

So, if you can get your ³dd bigfile² test to write data at 50MB/s+ with a
blocksize of 8KB, you should be doing well enough.

Incidentally, we also find that using the XFS filesystem and setting the
readahead to 8MB or more is extremely beneficial for performance with the
3Ware cards (and with others, but especially for the older 3Ware cards).

Regards,

- Luke