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SubjectRe: Kernel 2.6.17 and RAID5 Grow Problem (critical section backup)
On Friday July 7, jpiszcz@lucidpixels.com wrote:
>
> Hey! You're awake :)

Yes, and thinking about breakfast (it's 8:30am here).

>
> I am going to try it with just 64kb to prove to myself it works with that,
> but then I will re-create the raid5 again like I had it before and attempt
> it again, I did not see that documented anywhere!! Also, how do you use
> the --backup-file option? Nobody seems to know!

man mdadm
--backup-file=
This is needed when --grow is used to increase the number of
raid-devices in a RAID5 if there are no spare devices avail-
able. See the section below on RAID_DEVICE CHANGES. The file
should be stored on a separate device, not on the raid array
being reshaped.


So e.g.
mdadm --grow /dev/md3 --raid-disk=7 --backup-file=/root/md3-backup

mdadm will copy the first few stripes to /root/md3-backup and start
the reshape. Once it gets past the critical section, mdadm will
remove the file.
If your system crashed during the critical section, then you wont be
able to assemble the array without providing the backup file:

e.g.
mdadm --assemble /dev/md3 --backup-file=/root/md3-backup /dev/sd[a-g]

NeilBrown
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