[mrtg] Re: linux performance

[Steve Shipway]

Pavel Ruzicka wrote
> There are LSI Logic megaraid with two 15000rpm disks in a mirror.
> When everything is OK, MRTG runs for about 45 seconds.
> All RRD files are probably cached by kernel in a RAM. Next kernel
> writes caches to a disk for about 30 seconds on my machine. Now is
> machine totally dead. I don't know what with this situation.

We also have this problem, although it occurs a bit more randomly
because we spread the disk accesses over the 5-min window.  We have
tried a huge number of things to try and improve this without success -
the machine will lock up for about 10-30sec in order to flush the disk
caches when they become too full.

I think the only thing that can stop this is to have faster disks -
kernel tuning doesnt help because in the end it is caused by the OS just
having to wait for the disks to catch up.  Extra memory is good, of
course, but you won't ever have enough memory to hold ALL your RRD files
in cache permanently at once.

> 3. Optimized filesystem for many files. I have turned on  feature
"dir_index"
> on ext3 partition with data files. Next I have mounted  partition with
option
> "noatime". Maybe ext2 or other filesystem will be faster, but I don't
know...

I had forgotten that we had also done this.  The noatime mount option is
a good performance win, too.

> Have somebody tips for better multitasking, when kernel  flushes
caches?
> I tried tune kernel preempting and schedulers, but without  visible
change.

I'd like to know suggestions, too, but I think it all comes down to
getting the fastest disks and fastest inteface you possibly can.  I
looked at using our SAN for this (it has a very fast throughput) but
this makes our monitoring dependent on the SAN which has a very high
number of 'scheduled' outages...

Steve

--
Steve Shipway
ITSS, University of Auckland 
(09) 3737 599 x 86487
s.shipway at auckland.ac.nz

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