iostat can be a bit "special".I -think- under linux its the amount of time spent waiting for pending disk IO to complete. Now, some chipsets and their drivers seem to spend a lot of time in IOWAIT compared to others. The "traditional" difference was polled vs dma'ed disk IO - with polled IO, the driver would submit a request and then sit there and wait for it to complete. That time spent waiting was "IOWAIT". DMA'ing controllers would spend less time in IOWAIT because they'd submit a request, kick it off, it'd happen in the background, and then the kernel would be notified when it completed. Almost no time was spent in IOWAIT - just the time scheduling the DMA commands and handling the response.
Now, I've seen some low-ened SATA chipsets on some reasonably speccy hardware (eg some of the nvidia-driver Sun workstations) with large amounts of IOWAIT time. I don't know if its a driver thing or a hardware thing (or both), but there are definitely issues. Now, you could go groveling through the kernel to try and figure out whether IOWAIT includes "nfs activity" (which I believe would be the disk IO related to NFS, but it could be other things I guess) and first see if doing userspace disk IO does a lot of IOWAIT. Grab bonnie or some other hard disk throughput testing thing, run it, and see how much time is spent in IOWAIT. If you've got almost no IOWAIT time when doing it locally but IOWAIT time when doing it over NFS, you could be right. If you have IOWAIT time on both, I'd poke the disk/controller/ driver combo. 2d, Adrian On Thu, Oct 09, 2008, David Kempe wrote: > Grant Street wrote: > > > > > >I have a machine with a good proportion of IOWait 20-30%. It does have > >local disks and it performs operations on NFS mounts. I just wanted to > >be sure if IOWait includes NFS activity or not. I also want a way if it > >is NFS to be able to say for sure if it is a bottleneck on the nfs > >client or server. NFS is not a linux machine so visibility is not > >allways the best. > > > dstat might help you correlate stuff. > http://dag.wieers.com/home-made/dstat/ > > dave > -- > SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ > Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html -- - Xenion - http://www.xenion.com.au/ - VPS Hosting - Commercial Squid Support - - $25/pm entry-level VPSes w/ capped bandwidth charges available in WA - -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
