mikas blog

Today I wanted to test the raid5 reshape feature of Linux kernel >=2.6.17. My testing environment was VMware with 3 virtual disks each containing a partition of 200MB which should be resized to 500MB, using grml (featuring kernel 2.6.17-grml) of course. For real-live setup adjust the steps for stuff like switching harddisks of course. ;-)

# Initiate a RAID5 setup for testing purposes:
mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/hda1 /dev/hdb1 /dev/hdd1


# Create filesystem, mount md0, create a testfile and save md5sum for later check:
mkfs.ext3 /dev/md0
mount /dev/md0 /mnt/test
dd if=/dev/urandom of=/mnt/test/dd bs=512 count=10000
md5sum /mnt/test/dd > md5sum

# Make sure the RAID is synched via checking:
cat /proc/mdstat

# Now remove one partition:
mdadm /dev/md0 --fail /dev/hdd1 --remove /dev/hdd1

# Delete partition, create a new + bigger one and set partition type to fd (Linux raid autodetect):
cfdisk /dev/hdd

# And re-add the partition:
mdadm -a /dev/md0 /dev/hdd1

# Make sure the RAID is synched via checking:
cat /proc/mdstat

# Repeat the steps for all other disks/partitions as well:
mdadm /dev/md0 --fail /dev/hdb1 --remove /dev/hdb1
cfdisk /dev/hdb
mdadm -a /dev/md0 /dev/hdb1
cat /proc/mdstat
mdadm /dev/md0 --fail /dev/hda1 --remove /dev/hda1
cfdisk /dev/hda
mdadm -a /dev/md0 /dev/hda1
cat /proc/mdstat

Now we are ready to resize the RAID5 system online(!) [see ‘man mdadm’ for details]:

root@grml ~ # mdadm --detail /dev/md0 | grep -e 'Array Size' -e 'Device Size'
     Array Size : 390912 (381.81 MiB 400.29 MB)
    Device Size : 195456 (190.91 MiB 200.15 MB)
root@grml ~ # mdadm --grow /dev/md0 -z max
root@grml ~ # mdadm --detail /dev/md0 | grep -e "Array Size" -e 'Device Size'
     Array Size : 976512 (953.79 MiB 999.95 MB)
    Device Size : 488256 (476.89 MiB 499.97 MB)

Last step – resize the filesystem (online again):

root@grml ~ # df -h | grep test
/dev/md0              370M   15M  336M   5% /mnt/test
root@grml ~ # resize2fs /dev/md0
resize2fs 1.39 (29-May-2006)
Filesystem at /dev/md0 is mounted on /mnt/test; on-line resizing required
Performing an on-line resize of /dev/md0 to 976512 (1k) blocks.
The filesystem on /dev/md0 is now 976512 blocks long.

root@grml ~ # df -h | grep test
/dev/md0              924M   16M  861M   2% /mnt/test

Let’s check whether the testfile is still ok:

root@grml ~ # md5sum -c md5sum
/mnt/test/dd: OK

No downtime, no data lost. Excellent. :-) Thanks to Lars Schimmer for the idea and his basic instructions!

This entry was posted by mika on Saturday, September 9th, 2006 at 21:49 and is filed under Computer, English. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.