[mrtg] ext3 sync speed

[Steve Shipway]

>I use MRTG 2.16.4 with RRDTool 1.4.3 for about 30000 targets, and I have
>long-time troubles with blocked system while kernel do SYNC of all cached
>RRD files.

We also had this issue (40,000 targets) for a long time; tried tuning commit which helps a bit but not much.  Obviously the fastest disks you can get help to reduce the bottleneck.  RRD 1.4 helps a bit, though noatime makes little difference.

You get significant savings by making sure MRTG is using daemon mode (as opposed to using cron or similar) with RRD 1.4, so that the cfg files are cached and the memory-mapped features of RRD1.4 kick in.

The main thing that fixed it for us was using rrdcached.  However this requires you to use the latest version of MRTG, plus the trunk version of RRD 1.4 as well as the latest version of Routers2 (if you use this as the frontend) and it has a couple of other caveats.

rrdcached is the read/write RRD caching daemon that goes between rrdtool and the disk.  You can tune it to be more or less aggressive and this has a significant impact on disk I/O queues.

Steve

Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.shipway at auckland.ac.nz
Ph: +64 9 373 7599 ext 86487