Growing a Software Raid 5 array with XFS on it.

[Seth Mos <knuffie@xs4all.nl>]

Hi,

This is the log of a semi succesful conversion of a software raid 5 array 
with raidreconf from a 3 disk array to a 4 disk array.

raidreconf is a utility to add disks to a existing raid 0 or 5 array. In 
this case it involved a 3 disk raid 5 array with XFS (version 1 log) on it. 
The operating system was Red Hat Linux 7.3 Raidreconf was version 0.1.2.
The partitions were about 40GB each. The raid was about 77GB netto. The 
filesystem contained about 39GB of data.

The old raidtab entry looked like this:

# md0 is the /data raid array
raiddev                 /dev/md0
raid-level              5
nr-raid-disks           3
chunk-size              128
parity-algorithm left-symmetric
# Spare disks for hot reconstruction
nr-spare-disks          0
persistent-superblock   1
# Define 1st RAID disk
device                  /dev/hde1
raid-disk               0
# Define 2nd RAID disk
device                  /dev/hdg1
raid-disk               1
# Define 3rd RAID disk
device                  /dev/hdb1
raid-disk               2

The new one looked like this:

# md0 is the /data raid array
raiddev                 /dev/md0
raid-level              5
nr-raid-disks           4
chunk-size              128
parity-algorithm left-symmetric
# Spare disks for hot reconstruction
nr-spare-disks          0
persistent-superblock   1
# Define 1st RAID disk
device                  /dev/hde1
raid-disk               0
# Define 2nd RAID disk
device                  /dev/hdg1
raid-disk               1
# Define 3rd RAID disk
device                  /dev/hdb1
raid-disk               2
# Define 4th RAID disk
device                  /dev/hdc9
raid-disk               3

The partition tables looked something like this.
/dev/hde1             1     77207  38912296+  fd  Linux raid autodetect
/dev/hdg1             1     77207  38912296+  fd  Linux raid autodetect
/dev/hdb1             1      4972  39937558+  fd  Linux raid autodetect
/dev/hdc9         52014    131252  39936424+  fd  Linux raid autodetect

And yes I am very aware that putting 2 7200RPM IDE disks on one UDMA33 
channel is not the fastest thing to do.
New ide controller for two more ports arriving soon.

The procedure involves stopping the /dev/md0 raid array and start the 
conversion with:
raidreconf -o /etc/raidtab.old -n /etc/raidtab.new -m /dev/md0

All this went reasonably well but on the last few percent of the 
reconstruction raidreconf barfed. I don't have the exact error but it 
reached this error (by grepping the source):
rrc_raid5.c.backup:             fprintf "raid5_map_global_to_local: disk %d 
block out of range: %lu (%lu) gblock = %lu\n"
in this case the error was on block 60500 out of 61500 something. The disk 
was number 1.

Because this happened with just one hash mark to go I ad some hope that the 
filesystem would be mostly intact. The process already ran for about 2 
hours. I recreated the /dev/md0 array with the new raidtab and it started 
resyncing.

I checked the filesystem with xfs-repair -n and it reported a few directory 
entries and one which it would move to lost and found. I ran xfs_repair and 
mounted the filesystem, and sure enough just one dierctory with about 10 
files in it was damaged.

The next step was growing the filesystem to it's new size. This was easily 
done and without error using xfs_growfs.
The /data raid 5 is now 4 disks of 40GB each and the netto space is 111GB.
The growing is only and was done with the raid array still resyncing. This 
took about 10 seconds.

I got really lucky I guess. At least I don't have to restore a lot of data 
from backup.
AND YES YOU DO NEED BACKUPS. It's good that the manual for raidreconf 
explicitly states this and every webpage that mentions raidreconf.

It's all working now but this is one for the archives.

Cheers
--
Seth
It might just be your lucky day, if you only knew.