background fsck high load on 8.1

Hello,

we've experienced that background fsck on 8.1 degrades server performance on a higher degree than in previous fbsd versions (6.3, 7.3; amd64).

We've noticed it after upgrading - same hardware - to a 8.1-RELEASE.
Now, performance of other services (i.e. apache, mysql) during a background fsck falls miserably.

Is there any way to calm fsck down?, nice(1)?, some sysctl?

We have also gmirror, but we prevent to rebuild it if there is a fsck running in background.

Thanks for your help,
regards,
Sergi