OT: high WIO on Linux
- From: Mark Brinsmead <mark.brinsmead@xxxxxxx>
- To: oracle-l@xxxxxxxxxxxxx
- Date: Fri, 10 Mar 2006 00:05:11 -0700
Sorry if this is a bit off topic, but I imagine that a few of you may
have encountered this in the past...
Recently, I have observed a couple different Linux boxes (I'm reasonably
sure both were RHEL3) displaying very unusual-looking WIO statistics.
On one, I was seeing about 15% USER, and 80%+ WIO. (Okay, that's bad,
but not preposterous, except that 'iostat' showed all disks were
essentially idle!) The other was even weirder -- 50% IDLE and 50% WIO.
There was nothing there to generate I/O requests.
Both machines had (at least) one thing in common -- aside from running
Oracle. They each had *much* more RAM than SWAP. (RAM/SWAP was 6GB/2GB
for the first case, and 4GB/1GB for the second.)
Is it possible that all of these I/O waits are being generated inside
the kernel -- perhaps by the filesystem buffer cache trying to steal
pages from itself, or something equally unproductive.
By the way, in neither case was there noticeable evidence of paging or
swapping. The swap space itself was esentially unused...
Can anybody offer a plausible (even if it is hypothetical) explanation
for this behaviour? Or maybe direct me to some resources that will help
me better understand *how* or *why* such things can happen?
I mean, really! 50% IDLE + 50% WIO is awfully strange! My Linux skills
are maybe a bit "lightweight", but I've been working with UNIX for
decades and never seen (or at least never *noticed*) anything remotely
like this...
Cheers,
-- Mark.
--
//www.freelists.org/webpage/oracle-l
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