For our mirror server we now have a third RAID which is also used for the mirror data. The previous external RAIDs (12x1TB as RAID5 + hot spare) were reaching their limits and so additional 11x1TB as RAID6 in the remaining internal slots are a great help to reduce the load and usage of the existing disks. There are now roughly 30TB used for mirror data.
To create the filesystem on the new internal RAID I have used http://busybox.net/~aldot/mkfs_stride.html. With 11 disks, a RAID level of 6, RAID chunk size of 512 KiB and number of filesystem blocks of 4KiB I get the following command to create my ext4 filesystem:
mkfs.ext4 -b 4096 -E stride=128,stripe-width=1152
I am now moving all the data from one of the external RAIDs to the new internal RAID because the older external RAID still uses ext3 and I would like to recreate the filesystem using the same parameter calculation as above. Once the filesystem has been re-created I will distribute our data evenly across the three RAIDs (and maybe also mirror a new project).
Update: After moving the data from one of the external RAIDs to the internal RAID the filesystem has been re-created with:
mkfs.ext4 -b 4096 -E stride=128,stripe-width=1280