How can I restore an LVM snapshot of a larger volume on a smaller disk?

I am trying to backup/restore an LVM volume on a local Ubuntu 20.04 (Focal Fossa) server. The production server has an LV of 500 GB, however there are only 19 GB used so far.

The local development server has 24 GB free space where I intend to restore the 19 GB from production. I do use -L 24G as a parameter while doing the LVM snapshot. The process fails to restore with the message: "no space left on device":

Production Server:

sudo lvcreate -s /dev/vg0/test -n backup_test -L 24G
sudo dd if=/dev/vg0/backup_test  | lz4 > test_lvm.ddimg.lz4

1048576000+0 records in
1048576000+0 records out
536870912000 bytes (537 GB, 500 GiB) copied, 967.79 s, 555 MB/s

sudo lvdisplay /dev/vg0/backup_test
  --- Logical volume ---
  LV Path                /dev/vg0/backup_test
  LV Name                backup_test
  VG Name                vg0
  LV UUID                IsGBmM-VM7C-2sO4-VrC1-kHKg-EzcR-4Hej44
  LV Write Access        read/write
  LV Creation host, time leo, 2021-07-04 12:45:45 +0200
  LV snapshot status     active destination for m360
  LV Status              available
  # open                 0
  LV Size                500.00 GiB
  Current LE             128000
  COW-table size         24.00 GiB
  COW-table LE           6144
  Allocated to snapshot  0.01%
  Snapshot chunk size    4.00 KiB
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:10

Local Test Server:

sudo lvcreate -n restore -L 24.5G data
sudo mkfs.ext4 /dev/data/restore
sudo lz4 -d test_lvm.ddimg.lz4 | sudo dd of=/dev/data/restore

Warning : using stdout as default output. Do not rely on this behavior: use explicit `-c` instead !
dd: writing to '/dev/data/restore': No space left on device
51380225+0 records in
51380224+0 records out
26306674688 bytes (26 GB, 24 GiB) copied, 1181.48 s, 22.3 MB/s

Is there a way to restore the 500 GB volume containing only 19 GB on a smaller disk than 500 GB?

Solution 1:

The data you are trying to restore has a size of 500 GB, and that will not fit on a logical volume of size 24 GB. You can be more selective about what you're transferring, or you can allocate more space to the target volume group.

It sounds like the data on the logical volume you're backing up is only 19 GB. There are a few ways you can transfer that data.

If you only care about the data and not the filesystem, you could create a new filesystem (via mkfs.ext4 or so) on the target, and use tools that operate on files instead of block devices, like rsync(1) or tar(1).

Alternatively, you could use fsarchiver(8), which can back up a filesystem, and then restore it to a smaller device.

Finally, you could shrink the size of the block device you're backing up with resize2fs(8) and lvreduce(8). You can then back up that smaller volume, and restore to a similarly sized volume on the new server.

Solution 2:

The only way that you could restore your data is to use a block device that is compressed (e.g VDO, loop-device over a compressed file system like btrfs and ZFS).

However even if your data is only 17GB, the free space might contain data from previous writes and the entire LV might not be that compressible.

As compressed filesystems you can use btrfs or ZFS, and create a 500G zeroed file that you map to a loop device (with losetup), or, on ZFS, you can create directly a block device that is compressed. Then restore your LV to the block device you created in previous step. Compression should give you a chance to restore all data.