Re: Understanding I/O behaviour

["Daniel J Blueman" <daniel.blueman@gmail.com>]

> > On 5 Jul, 16:50, Martin Knoblauch <spamtrap@knobisoft.de> wrote:
> > > Hi,
> > >
> > >  for a customer we are operating a rackful of HP/DL380/G4 boxes
> > that
> > > have given us some problems with system responsiveness under [I/O
> > > triggered] system load.
> > [snip]
> >
> > IIRC, the locking in the CCISS driver was pretty heavy until later in
> > the 2.6 series (2.6.16?) kernels; I don't think they were backported
> > to the 1000 or so patches that comprise RH EL 4 kernels.
> >
> > With write performance being really poor on the Smartarray
> > controllers
> > without the battery-backed write cache, and with less-good locking,
> > performance can really suck.
> >
> > On a total quiescent hp DL380 G2 (dual PIII, 1.13GHz Tualatin 512KB
> > L2$) running RH EL 5 (2.6.18) with a 32MB SmartArray 5i controller
> > with 6x36GB 10K RPM SCSI disks and all latest firmware:
> >
> > # dd if=/dev/cciss/c0d0p2 of=/dev/zero bs=1024k count=1000
> > 509+1 records in
> > 509+1 records out
> > 534643200 bytes (535 MB) copied, 11.6336 seconds, 46.0 MB/s
> >
> > # dd if=/dev/zero of=/dev/cciss/c0d0p2 bs=1024k count=100
> > 100+0 records in
> > 100+0 records out
> > 104857600 bytes (105 MB) copied, 22.3091 seconds, 4.7 MB/s
> >
> > Oh dear! There are internal performance problems with this
> > controller.
> > The SmartArray 5i in the newer DL380 G3 (dual P4 2.8GHz, 512KB L2$)
> > is
> > perhaps twice the read performance (PCI-X helps some) but still
> > sucks.
> >
> > I'd get the BBWC in or install another controller.
> >
> Hi Daniel,
>
>  thanks for the suggestion. The DL380g4 boxes have the "6i" and all
> systems are equipped with the BBWC (192 MB, split 50/50).
>
>  The thing is not really a speed daemon, but sufficient for the task.
>
>  The problem really seems to be related to the VM system not writing
> out dirty pages early enough and then getting into trouble when the
> pressure gets to high.

Hmm...check out /proc/sys/vm/dirty_* and the documentation in the
kernel tree for this.

Just measuring single-spindle performance, it's still poor on RH EL4
(2.6.9) x86-64 with 64MB SmartArray 6i (w/o BBWC):

# swapoff -av
swapoff on /dev/cciss/c0d0p2

# time dd if=/dev/cciss/c0d0p2 of=/dev/null bs=1024k count=1000
real    0m49.717s  <-- 20MB/s

# time dd if=/dev/zero of=/dev/cciss/c0d0p2 bs=1024k count=1000
real    0m25.372s  <-- 39MB/s

Daniel
--
Daniel J Blueman
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