LVM is an amazingly useful tool, however, it seems to still lack support for snapshotting a drive that is already a snapshot. I made a script to handle this automatically but ran into some troubles.

My test environment

I'm testing the script on a clean Xen-box (from now on:testbox). After creating the box I created a new LVM volume and added it to the testbox's drives. In testbox itself, it is displayed as a normal block device so I don't think the Dom0's LVM should interfere with the testing process.

The original drive

On testbox, i created a new partition using the following commands:

# Using the data in the other tables i determined # where i could begin my new device sectors dmsetup table # Create the new device without a table dmsetup create base --notable # Put the table into the device... echo '0 4194304 linear 202:2 0' | dmsetup load base dmsetup resume base mkfs.ext2 /dev/mapper/base 

To be clear, the second target argument '202:2' is that second device I added to the testbox machine, I double checked it like so:

ls /dev -l | grep 'xvda2' 

Returning:

brw-rw---- 1 root disk 202, 2 May 3 17:01 xvda2

The script

I wrote this function to make a snapshot:

function create_dm_snapshot { banner "0: Checking if block devices don't allready exist, original device should exist..."; device_exists $base_path$original; [ $? -eq 0 ] || error 'The source (original) device should exist'; device_exists $base_path$snapshot_origin $base_path$snapshot $base_path$cow; [ $? -eq 0 ] && error "They allready exist pls use the 'remove' function"; echo "Done checking."; banner "1: Suspending the original device."; suspend_dev $original || error "Failed suspending original device"; banner "2: Creating snapshot-origin."; create_dev $snapshot_origin || error "Failed creating snapshot-origin"; banner "3: Read original table into snapshot-origin."; dmsetup table $original | dmsetup load $snapshot_origin || error 'Failed loading original table into snapshot-origin'; echo "Done reading."; banner "4: Resume snapshot-origin."; resume_dev $snapshot_origin || error 'Could not resume snapshot-origin'; banner "5: Create snapshot device."; create_dev $snapshot || error 'Failed to create snapshot device'; banner "6: Create COW-device."; #TODO: check total sector count device create_dev $cow ; target_device=$( dmsetup table $original | awk '{print $4}' ); last_table=$( dmsetup table | grep "$target_device" | awk '{print $6}' | sort -g | tail -n 1 ); begin_sector_args=( $( dmsetup table | grep -E $target_device".*"$last_table"|"$last_table".*"$target_device | awk '{print $2 " " $3 " " $6}' ) ); begin_sector=$( expr ${begin_sector_args[1]} - ${begin_sector_args[0]} + ${begin_sector_args[2]} ); table="0 $size linear $target_device $begin_sector"; echo $table | dmsetup load $cow; resume_dev $cow; banner "7: Calculate rowcount in snapshot-origin"; snapshot_origin_size=$( blockdev --getsz $base_path$snapshot_origin ) || error 'Could not determine rowcount'; echo "Snapshot size: $snapshot_origin_size"; banner "8: Load snapshot table."; table="0 $snapshot_origin_size snapshot $base_path$snapshot_origin $base_path$cow p 64"; [ $verbose ] && echo "Table: $table"; echo $table | dmsetup load $snapshot || error 'Failed loading snapshot table'; echo "Done loading."; banner "9: Reload original device table."; table="0 $snapshot_origin_size snapshot-origin $base_path$snapshot_origin"; [ $verbose ] && echo "Table: $table"; echo $table | dmsetup load $original || error 'Failed reloading original table'; echo "Done reloading."; banner "10: Resume frozen tables."; resume_dev $snapshot $original || error 'Could not resume devices'; echo "Done resuming."; } 

The error

At step 8 (banner "8: ...) the script fails with the following error:

device-mapper: reload ioctl failed: No such device or address

Command failed

,

dmsetup table 

Results in the following table data:

dm.base.snapshot_origin: 0 4194304 linear 202:2 0

base: 0 4194304 linear 202:2 0

dm.base.snapshot:

dm.base.cow: 0 4096 linear 202:2 4194304

As I wasn't able to determine the cause of the error the last step I did was look into my dmesg...

dmesg | tail Giving me: 

PM: freeze of devices complete after 0.080 msecs

suspending xenstore...

PM: late freeze of devices complete after 0.019 msecs

PM: early restore of devices complete after 0.035 msecs

PM: restore of devices complete after 32.367 msecs

Setting capacity to 10485760

Setting capacity to 104857600

device-mapper: persistent snapshot: Invalid or corrupt snapshot

device-mapper: table: 254:2: snapshot: Failed to read snapshot metadata

device-mapper: ioctl: error adding target to table

I wasn't able to find out what caused the snapshot to be corrupt.