NeilBrown wrote:
Does the mdadm --detail output I pasted in my last message hold any clues?
Yes. Based on the "used device size", the smallest device was between
242244096K and 242244160K
Hopefully both of the smaller devices were the same size.
They were (the same size) but I couldn't find the superblock. I tried
several possible partition sizes around 242244032K (plus or minus a
cylinder or two) and no luck.
In the end I gritted my teeth and, following the advice in
http://joshuahayes.blogspot.com/2006/11/expand-existing-raid-5-array.html
I forced mdadm --create to recreate the array using correct parameters.
mdadm --create /dev/md0 --chunk=64 --level=5 --layout=left-symmetric
--raid-devices=4 --size=242244032 /dev/sda2 /dev/sdb2 /dev/sdd2 /dev/sdc2
and after a reboot, everything came all the way up to multiuser (which
is significant, because /usr was on an LVM on this RAID partition) and
as far as I can see, everything is fine.
Phew!
As to where my superblock has gone, the only theory I have is that the
MD layer knew that my partitions were 400G large while the kernel was
convinced they were 250G large, so the md layer tried to write the
superblock at (approx) +400G, and the kernel refused to do that.
However, since the used dev size was only 242244032, all my actual data
was safe and just using mdadm to recreate the superblocks was all that
was needed.
The lesson here is always reboot after changing partition sizes, unless
you have a tool which reliably flushes the kernel partition table cache
(partprobe?)
Thanks for the help,
Jules
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