Make snapshot of non-busy block device with device mapper in Linux

make-snapshot-of-device.sh

NAME=nameofsnapshot
DEVRO=/dev/read-only-backing-device
DEVCOW=/dev/space-for-writing-changes

SECTORS=$(blockdev --getsz $DEVRO)
TABLE=$(mktemp)

echo 0 $SECTORS snapshot $DEVRO $DEVCOW N 16 >> $TABLE

echo "Setting up $NAME with"
head $TABLE

dmsetup create $NAME < $TABLE

rm $TABLE

N = not persistent, will not survive reboot
16 = chunk size in sectors for CoW, here 8 KiB

Query status: dmsetup status $NAME
Remove snapshot: dmsetup remove $NAME

The backing device must not be "busy". Testing confirms this includes "mounted" but may also include other uses such as being an active PV of LVM, opened with cryptsetup or a MD RAID member.

References:

• https://blog.oddbit.com/post/2018-01-25-fun-with-devicemapper-snapshot/
• https://www.kernel.org/doc/Documentation/device-mapper/snapshot.txt

Hi,
I tried this recipe (and many more from the net) but I get always the same error:
"device-mapper: reload ioctl on snap-sdb1 (253:0) failed: Device or resource busy
Command failed."
and "dmsetup status" says "No devices found". This is on a debian11 VirtualBox PC.
As always: any help appreciated, Hans

Thanks for reporting the issue. Trying it with and without mounting the backing device I get the same error message when the device is mounted, suggesting that dmsetup create cannot use mounted block devices. "Busy" probably also applies to other scenarios such as the backing device being part of an active software RAID. Will update the description.

Hi Joachim, thank's a lot, it now works. Kind regards & seasons greetings, Hans

On 21.12.2022 14:23, Joachim Wagner wrote: @jowagner commented on this gist. ------------------------- Thanks for reporting the issue. Trying it with and without mounting the backing device I get the same error message when the device is mounted, suggesting that dmsetup create cannot use mounted block devices. "Busy" probably also applies to other scenarios such as the backing device being part of an active software RAID. Will update the description. -- Reply to this email directly, view it on GitHub [1] or unsubscribe [2]. You are receiving this email because you commented on the thread. Triage notifications on the go with GitHub Mobile for iOS [3] or Android [4].

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