[CentOS] disk I/O problems and Solutions

Alan McKay alan.mckay at gmail.com
Fri Oct 9 17:34:17 UTC 2009

On Fri, Oct 9, 2009 at 1:28 PM, Ray Van Dolson <rayvd at bludgeon.org> wrote:
> Really hard to say what's going on.  Does your DB need optimization?
> Do the applications hitting it?  Maybe some indexing?  Maybe some more
> RAM on the machine would help?  What exactly is the workload like --
> especially during the time when you're peaked out?

Yes, these are all things we are just starting to look at - so no
clear answers yet

> Is the system swapping?  If so, you either need more memory or need to
> track down a memory leak.... 'free' and 'sar' can both help you see
> what swap usage is like.

That's one thing it does not seem to be doing, fortunatley.

> Writes are always slower on any parity based RAID setup, so I imagine
> you'd get superior performance on RAID10, especially if you're write
> heavy.

We are generally read heavy but at certain times of day including 6 to
9am we have huge batches of writes.

> But to begin with, it'd be interesting to know exactly what this server
> is doing.  Does it makes sense that the disks are being brought to
> their knees with the given workload?

Oh yes!   We are a financial services provider and the Db contains all
of the worlds stock prices for about the last 10 years :-)  And every
day new batches of prices come in (at different times depending on
markets).   This system is just a PostgreSQL DB.   Nothing else.

> Is the disk array you bought an N-series? (N3300, N3600)?  If so, those
> are NetApps and should be quite fast thanks to heavy write caching.
> Even then, you'll be limited by spindles it sounds like...

No, the array is IBM brand.   EXP3000 (I've since looked it up)

“Don't eat anything you've ever seen advertised on TV”
         - Michael Pollan, author of "In Defense of Food"

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