Louwrentius

As described earlier, I setup a RAID 6 array consisting of physical 1 TB disk and 'virtual' 1 TB disks that are in fact two 0.5 TB disks in RAID 0. 

I wanted to upgrade to Lenny because the new kernel that ships with Lenny supports growing a RAID 6 array. After installing Lenny the RAID 0 devices were running smootly, but not recognised as part of the RAID 6. 

So the array was running in degraded mode. That is bad.

In Lenny, a new version of mdadm is used that requires the presense of the mdadm.conf file. The mdadm.conf file contains these lines: 

#DEVICE partitions
#DEVICE /dev/md*

After I uncommented the "DEVICE /dev/md*" line and generated a new initramfs file with:

update-initramfs -u

The RAID 0 drives were recognised as part of a RAID array and everything was OK again. So mdadm must be instructed to check if /dev/md? devices are a member of a RAID array. 

I guess this is also relevant if you are running a RAID 10 based on a mirrored stripe or a striped mirror.

When formatting a RAID device with an EXT filesystem, it is always advised to specify a stride size. The format utility will take this stride size into account when formatting a device. The stride size is the number you get when you divide the 'chunck' size, as specified with MDADM by the filesystem block size (almost always 4K).

So a 128 KB chunck size gives you a stride of 32. A nice and simple utility to calculate your stride can be found here:

http://busybox.net/~aldot/mkfs_stride.html

Please note that the stripe-with option does not seem to work on Debian Etch. Maybe because that option is too old or too new.

I also think that most recent tools automatically detect and calculate the correct stride size for you.