>
> Step 1 in all this is, of course, to take a backup. And step 2 is to
> check that your backup is good.
Everything important was backed up recently. :-)
>
> It is also a good idea to practice on fake arrays made from loopback
> "disks" - they work fine for md raid, and let you practice re-shaping,
> re-sizing, etc., without any risk to your real disks.
I also recommend this for things I am not sure about,
and I imagine it is critical for development testing.
In fact, when I like to give a demo to folks:
--I'll first create a RAID array with loop devices.
--Ill then copy over a movie and start watching the movie.
--I'll then fail a disk and delete the file
--Then I'll create a new file, add it to the array
and watch it recover, all the while the movie doesn't skip a beat. :-)
>
>
> If you want to safely replace the disks in a raid5 array, the easiest
> way is to add a new disk (this can be an external USB disk if necessary)
> and re-shape to an asymmetric raid6 with parity Q on the new disk. Now
> you have an extra redundancy for safety. (Use asymmetric raid6 to avoid
> re-striping the existing disks.)
>
I would disagree with your assertion that it is the 'easiest' way.
I tried to do this on my system, but you didn't specify which parity
method to use.
RTFMing I found...
These same layouts are available for RAID6. There are also 4
layouts that will provide an intermediate stage for converting
between RAID5 and RAID6. These provide a layout which is iden‐
tical to the corresponding RAID5 layout on the first N-1
devices, and has the 'Q' syndrome (the second 'parity' block
used by RAID6) on the last device. These layouts are: left-sym‐
metric-6, right-symmetric-6, left-asymmetric-6, right-asymmet‐
ric-6, and parity-first-6.
Regardless, attempting to use any of these layouts were countered with
"that parity mode isn't available for that level raid." or something similar.
It was much 'easier' to compile the latest mdadm and use --replace
(Thanks Mikael):
apt-get install git
git clone git://neil.brown.name/mdadm
make install
mdadm --add /dev/md5 /dev/sdX
mdadm --replace /dev/md5 /dev/sdY
Recovery has been the same >100MB/sec and it is reading from one disk
'Y', writing to another 'X' without a full resync
Thanks again everyone your awesome ideas!
Now, time to see how many of these Linux raid options exist in my new
Lenovo/Iomega ix4 NAS unit.
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=lenovo%20ix4
Apparently it runs a flavor of Debian for ARM. I suspect I'll be
cross compiling some modern versions of our favorite tool :-)
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