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.
