Disk accesses freeze for a lot of seconds

[Federico Giannici]

We have an OpenBSD 5.2 amd64 where every 5 minutes a few thousand of 
".rrd" files from MRTG are written (actually, updated) to disk.

The problem is that for a few seconds (15-20) every other access to the 
disk is totally blocked. So during those 15-20 seconds the access to the 
graphs is freezed! And this is really annoying for a graphs server...

It's not a problem of CPU load (it's a quadruple core AMD Athlon II X4 
630 Processor). Processes run smoothly, they freeze only when they try 
to access the disk. Disk is a normal SATA, and the partition is FFS with 
softdep.

Is there anything (some system tuning?) I can do to get rid of the 
freezes, or at least to mitigate them?

Thanks.