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

Hi Luke,

On Tue, 2005-11-15 at 10:42 -0800, Luke Lonergan wrote:
> With RAID5, it could matter a lot what block size you run your “dd
> bigfile” test with. You should run “dd if=/dev/zero of=bigfile bs=8k
> count=500000” for a 2GB main memory machine, multiply the count by
> (<your mem>/2GB).
If I understand correctly (I have 4GB ram):

jkr(at)Panoramix:~/tmp$ dd if=/dev/zero of=bigfile bs=8k count=1000000
1000000+0 records in
1000000+0 records out
8192000000 bytes transferred in 304.085269 seconds (26939812 bytes/sec)

Which looks suspicious: 26308 MB/sec???

> It is very important with the 3Ware cards to match the driver to the
> firmware revision.
OK, I am running 1 driver behind the firmware.

> 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, there’s no harm, but I’ve found that when you
> reach the point of experimenting with schedulers, you are probably not
> addressing the real problem.
It depends. I/O Schedulers (I assume) have a purpose: some schedulers
should be more appropriate for some circumstances. And maybe my specific
circumstances (converting a database with *many updates*) is a specific
circumstance. I really don't know....

> 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.
As I have an (almost) seperate WAL disk: iostat does not show any
significant writing on the WAL disk....

> So, if you can get your “dd bigfile” test to write data at 50MB/s+
> with a blocksize of 8KB, you should be doing well enough.
See above.

> 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).
I don't have problems with my read performance but *only* with my
*update* performance (and not even insert performance). But than again I
am not the only one with these problems:

http://www.issociate.de/board/goto/894541/3ware_+_RAID5_
+_xfs_performance.html#msg_894541
http://lkml.org/lkml/2005/4/20/110
http://seclists.org/lists/linux-kernel/2005/Oct/1171.html

I am happy to share the tables against which I am running my checks....

--
Groeten,

Joost Kraaijeveld
Askesis B.V.
Molukkenstraat 14
6524NB Nijmegen
tel: 024-3888063 / 06-51855277
fax: 024-3608416
e-mail: J(dot)Kraaijeveld(at)Askesis(dot)nl
web: www.askesis.nl