[mrtg] Re: Graphing lots of ports

Pavel Ruzicka pavouk at pavouk.org
Wed Dec 8 15:51:31 MET 2004


> >I have a pair of Linux machines graphing a lot of switch ports
> >(~35000 ports across 1500 devices each) on 5-minute intervals.
> >Disk I/O seems to be my biggest problem. The CPU load tends to

I have few additional commentaries:

> We have a similar problem.  I find these help:
> 1) FAST DISKS. Get the fastest disk IO you can.  Nice quick SCSIs or SAN.
> IDE just doesn't cut it.

I use two 15000RPM Ultra320 SCSI disks in a mirror.
I use MegaRAID SCSI controller.

> 2) Mount the filesystem with noatime option.  This speeds it up a lot as
> the atime flag doesn't have to be updated.

It's good tip.

> 3) Splt RRDs into multiple filesystems.  A big directory is a timewaster.
> This is less significant, though.

You can try tune filesystem. I enabled "dir_index" on ext3(2) filesystem which
resolve problems with big directories. This can be done on live filesystem 
with tune2fs.

dumpe2fs /dev/sda4 | less
Filesystem features:      has_journal dir_index filetype needs_recovery 

> 4) Lots of memory.  UNIX will use all spare memory for disk cache.

I have 3GB of RAM. All files are cached in memory during measuring.
I have about 8000 targets. MRTG runs for about 30 seconds, when all
targets are available. Flushing cache to disk takes about 15 seconds.
I use Linux with kernel 2.6.9. momentally.

Best regards,

Pavel Ruzicka

> 5) Use RRDTool, not log files!  Of course, you are doing this already.
> 6) Tune the filesystem for smaller files and small block IO. You never need
> more than the size of an RRD to be loaded, but you want it to block-load
> the whole RRD file.

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