With my Linux SW RAID screaming “grow me!” for quite a while now, I finally brought myself to replace the old 2TB disks with new 6TB ones (RAID 5 with 4 disks). While such a disk-upgrade has to be performed regularly, the frequency is so low that it is hard to remember the details when you finally get to do it again. Unfortunately the “official” method (replace & resync disk-by-disk and then grow the md and the filesystem) as suggested in the Linux RAID Wiki has a few drawbacks:

• you have no backup in case of failures during the 4 RAID rebuilds
• you continue to operate on the old filesystem, in my case where the RAID has been full for quite a while you will inherit quite a bit of unnecessary fragmentation – and you cannot switch nor re-tune the filesystem which could make sense for a significantly bigger RAID

Luckily Adrian reminded me of mdadm’s missing parameter, so I could perform this alternate RAID upgrade which I’ll detail below (should come in handy for my next upgrade). Continue reading →