quota question

Hi,

can somebody help me, where can I information about mtab, quota.conf files? The man pages from quota, repquota,... hasnt detailed info.
If I start the quotas, I become the next failure (I dont no, if it is running or not.)

[root@mail root]# quotaon -a
quotaon: using /var/aquota.group on /dev/md0: Invalid argument

I have RAID mirroring under /dev/md. The aquota.users file has any size, but aquota.group file has only zero length.

Thanx
Gábor