Matthew Nuzum wrote:
Jack Orenstein <[EMAIL PROTECTED]> writes:

I'm looking at one case in which two successive transactions, each
updating a handful of records, take 26 and 18 *seconds* (not msec) to
complete. These transactions normally complete in under 30 msec.

None of this is necessarily going to fix matters for an installation
that has no spare I/O capacity, though.  And from the numbers you're
quoting I fear you may be in that category.  "Buy faster disks" may
be the only answer ...

I had a computer once that had an out-of-the-box hard drive configuration
that provided horrible disk performance.  I found a tutorial at O'Reilly
that explained how to use hdparm to dramatically speed up disk performance
on Linux.  I've noticed on other computers I've set up recently that hdparm
seems to be used by default out of the box to give good performance.

Maybe your computer is using all of it's I/O capacity because it's using PIO
mode or some other non-optimal method of accessing the disk.

There's certainly some scope there. I have an SGI Octane whos SCSI 2 disks were set-up by default with no write buffer and CTQ depth of zero :/ IDE drivers in Linux maybe not detecting your IDE chipset correctly and stepping down, however unlikely there maybe something odd going on but you could check hdparm out. Ensure correct cables too, and the aren't crushed or twisted too bad.... I digress...

Assuming you're running with optimal schema and index design (ie you're not doing extra work unnecessarily), and your backend has better-then-default config options set-up (plenty of tips around here), then disk arrangement is critical to smoothing the ride.

Taking things to a relative extreme, we implemented a set-up with issues similar sounding to yours. It was resolved by first optimising everything but hardware, then finally optimising hardware. This served us because it meant we squeezed as much out of the available hardware, before finally throwing more at it, getting us the best possible returns (plus further post optimisation on the new hardware).

First tip would to take your pg_xlog and put it on another disk (and channel). Next if you're running a journalled fs, get that journal off onto another disk (and channel). Finally, get as many disks for the data store and spread the load across spindles. You're aiming here to distribute the contention and disk I/O more evenly to remove the congestion. sar and iostat help out as part of the analysis.

You say you're using IDE, for which I'd highly recommend switching to SCSI and mutliple controllers because IDE isn't great for lots of other reasons. Obviously budgets count, and playing with SCSI certainly limits that. We took a total of 8 disks across 2 SCSI 160 channels and split up the drives into a number of software RAID arrays. RAID0 mirrors for the os, pg_xlog, data disk journal and swap and the rest became a RAID5 array for the data. You could instead implement your DATA disk as RAID1+0 if you wanted more perf at the cost of free space. Anyway, it's certainly not the fastest config out there, but it made all the difference to this particular application. Infact, we had so much free I/O we recently installed another app on there (based on mysql, sorry) which runs concurrently, and itself 4 times faster than it originally did...

YMMV, just my 2p.


Rob Fielding
[EMAIL PROTECTED] Development Designer Servers Ltd

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