Thank you for the response..

quoted

Does Linux MD RAID support a method of hot replacing a disk WITHOUT
having to resort to degraded mode?

Yes, it does, if you use a recent kernel + mdadm

I remembered reading about this once, and researching once before, but
I am pretty sure my Xubuntu 13.10 distro doesn't have the flavor of
mdadm I need.  (Awesome work on this tool Neil! With mdadm you have
transformed the utility of the md subsystem, and made it almost
impossible to break an array with bad options)

However, you have another option anyway. Just remove the hot spare,
re-partition as needed, then grow the raid5 to raid6.
1) Wait for the re-sync to complete
2) Drop another old drive from the array
3) Re-partition
4) Add back to the array and re-sync
You will never have worse redundancy than current during the above process.
Personally, I'd probably use the hot spare to move to RAID6, and then use
the migration to move a drive to its replacement (assuming you have another
spare drive available).

Thanks for pointing out that obvious solution, I had almost forgot! I
think this had crossed my mind at some point, but I wasn't sure if I
needed RAID6 at this time. The 2TB Samsung F4EG drives have a 1e^15
BER, which is on par with enterprise drives. I've been using them for
about 3 years, and they are still barely audible and perform great. I
have even purchased several used (The genuine Samsung article, Made in
Korea, not the post-merger Seagate flavor) and they all seem to behave
awesome.
The other kink to this solution is that I only plan to have 4 drives
in the system when all is said and done. I might just go with a RAID10
in that case.

If I decide to go this route, migrating to RAID6 is certainly a great solution.

quoted

However, in my situation, my RAID5 partitions start in the middle of
the drive, complicating that slightly... Fortunately, I have a spare
drive or two to assist.
1) Stop RAID array
2) Clone one of the RAID devices to a larger disk (Using dd)
3) Remove the old RAID device from the system
4) Restart the RAID array in readonly mode (to test that the clone was
successful without marking the array as dirty, otherwise, revert to
the removed disk)
5) Optional: Restart the RAID array in readwrite mode to confirm
6) Repeat 1-5 for each additional disk
7) Grow the array (Resync starts at the new space)
8) Grow the filesystem

I did start this process and migrated the first drive. Array downtime
was acceptable to me.. Details:
1) I stopped the RAID array
2) I created a partition on my spare drive
   (starting at sector 2048 so my 4K sector drive lies on a 4K boundary)
3) I cloned the partition with dd, It ran for a few hours at 100MB/min
sustained:
dd if=/dev/sda2 of=/dev/sdf1 bs=1M
(in another terminal: "while killall -USR1 dd; do sleep 60; done" was
pretty handy for monitoring progress)
4) I couldn't figure out how to start the array readonly, but I
assembled it manually with the following:
mdadm --assemble /dev/md5 /dev/sdf1 /dev/sdb2 /dev/sdc2 /dev/sdd2
/dev/sde2 --no-degraded
mdadm: /dev/md5 has been started with 5 drives.

So, while this solution does require a spare disk, this is an option
for migrating raid5 without running the array in degraded mode.

Actually, I was trying to find the URL to show the migrate options, but
couldn't seem to find any docs in the mdadm wiki at:
http://vger.kernel.org/vger-lists.html#linux-raid
Also, the debian raid wiki, the Neil Brown blog, and various other
resources. Hopefully someone else will be able to provide the relevant link.
Perhaps searching the mailing list itself would be best (I definitely recall
seeing it discussed here), but I'm out of time now. Good luck.

I recall seeing it at one point too.. Maybe it was in the btrfs man pages?
Thanks again!