I'd missed the mdadm --grow feature as well, so I checked into it.
It is only capable of increasing size on raid5, not component count. The specific use case used as an example is that you slowly retire component drives and the replacements are larger. When all components are the larger size, you can grow the raid5 array to use the full size of the device, followed by a filesystem expansion to use the grown array.
That makes sense, given the disk layout of raid5 - its not hard to add more stripes on the end of components, but adding new components requires each stripe to change significantly.
To grow component count on raid5 you have to use raidreconf, which can work, but will toast the array if anything goes bad. I have personally had it work, and not work, in different instances. The failures were not necessarily raidreconf's fault, but its not fault tolerant is the point, as it starts at the first stripe, laying things out the new way, and if it doesn't finish, and finish correctly, you are in an irretrievable inconsistent state.
Bah, too bad.
I don't need it yet, but at some stage I'd like to be able to add another 250GB drive(s) to me array and grow the array to use the additional space in a safe/failsafe way.
Perhaps by the time I come to need it this might be possible?
R. -- http://robinbowes.com
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