Luke Lonergan wrote:
Alan,
On 11/18/05 9:31 AM, "Alan Stange" <stange@xxxxxxxxxx> wrote:
Here's the output from one iteration of iostat -k 60 while the box is
doing a select count(1) on a 238GB table.
avg-cpu: %user %nice %sys %iowait %idle
0.99 0.00 17.97 32.40 48.64
Device: tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sdd 345.95 130732.53 0.00 7843952 0
We're reading 130MB/s for a full minute. About 20% of a single cpu was
being used. The remainder being idle.
Cool - thanks for the results. Is that % of one CPU, or of 2? Was the
system otherwise idle?
Actually, this was dual cpu and there was other activity during the full
minute, but it was on other file devices, which I didn't include in the
above output. Given that, and given what I see on the box now I'd
raise the 20% to 30% just to be more conservative. It's all in the
kernel either way; using a different scheduler or file system would
change that result. Even better would be using direct IO to not flush
everything else from memory and avoid some memory copies from kernel to
user space. Note that almost none of the time is user time. Changing
postgresql won't change the cpu useage.
One IMHO obvious improvement would be to have vacuum and analyze only do
direct IO. Now they appear to be very effective memory flushing tools.
Table scans on tables larger than say 4x memory should probably also use
direct IO for reads.
We've done nothing fancy and achieved results you claim shouldn't be
possible. This is a system that was re-installed yesterday, no tuning
was done to the file systems, kernel or storage array.
Are you happy with 130MB/s? How much did you pay for that? Is it more than
$2,000, or double my 2003 PC?
I don't know what the system cost. It was part of block of dual
opterons from Sun that we got some time ago. I think the 130MB/s is
slow given the hardware, but it's acceptable. I'm not too price
sensitive; I care much more about reliability, uptime, etc.
What am I doing wrong?
9 years ago I co-designed a petabyte data store with a goal of 1GB/s IO
(for a DOE lab). And now I don't know what I'm doing,
Cool. Would that be Sandia?
We routinely sustain 2,000 MB/s from disk on 16x 2003 era machines on
complex queries.
Disk?! 4 StorageTek tape silos. That would be .002 TB/s. One has to
change how you think when you have that much data. And hope you don't
have a fire, because there's no backup. That work was while I was at
BNL. I believe they are now at 4PB of tape and 150TB of disk.
-- Alan
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