/
Assemble zero-copy disk image on Linux

Assemble zero-copy disk image on Linux

Owned by Martin Raiber
Last updated: Mar 13, 2017

If you backup using cow-raw image files on Linux (using btrfs or ZFS) you can assemble disk images without making copies of them. Those disk images can then be directly used as KVM or Xen disk or exported via iSCSI. UrBackup backs up volume images and not disk images. That is why this assemble step is required.

Create (writeable) snapshots of the volumes involved with btrfs or ZFS

E.g. with btrfs:

btrfs subvol snap 170302-2131_Image_SYSVOL 170302-2131_Image_SYSVOL_rw
btrfs subvol snap 170302-2132_Image_C 170302-2132_Image_C_rw

Create a boot sector image

Create a file somewhere in which the boot sector is stored. Links to the urbackuprestoreclient are in How to restore via command line.

truncate -s 16M /tmp/mbr.img
losetup /dev/loop0 /tmp/mbr.img
./urbackuprestoreclient --restore-mbr /media/backup2/urbackup/win2012server/170302-2132_Image_C_rw/Image_C_170302-2132.raw.mbr -o /dev/loop0

Inspect the partition layout using e.g. parted (ignore errors about the partitions ending beyond the block device). Use the "unit s" command to display the positions as sectors (512 byte blocks):

> unit s
> p
Model: Loopback device (loopback)
Disk /dev/loop0: 32768s
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags:

Number  Start    End         Size        Type     File system  Flags
 1      2048s    718847s     716800s     primary               boot
 2      718848s  188741631s  188022784s  primary

Resize the volume files to (at least) the partition sizes

UrBackup backs up only the parts actually used by the file systems, so the image files may be smaller than the partition sizes. We can easily make sure the sizes are correct by extending the files a bit:

ls -l /media/backup2/urbackup/win2012server/170302-2132_Image_C_rw/Image_C_170302-2132.raw
-rwxr-x--- 1 urbackup urbackup 96268189184 Mar  2 21:45 /media/backup2/urbackup/win2012server/170302-2132_Image_C_rw/Image_C_170302-2132.ra

So image size is 188022783s (96268189184/512-1024 – volumes start at a 512kB offset into the file) but needs to be at least 188022784s. So we need to add at least one sector:

truncate -s +512 /media/backup2/urbackup/win2012server/170302-2132_Image_C_rw/Image_C_170302-2132.raw
ls -l /media/backup2/urbackup/win2012server/170302-2131_Image_SYSVOL_rw/Image_SYSVOL_170302-2131.raw
-rwxr-x--- 1 urbackup urbackup 367525376 Mar  2 21:32 /media/backup2/urbackup/win2012server/170302-2131_Image_SYSVOL_rw/Image_SYSVOL_170302-2131.raw

Image size is 716799s so we need to add one sector:

truncate -s +512 /media/backup2/urbackup/win2012server/170302-2131_Image_SYSVOL_rw/Image_SYSVOL_170302-2131.raw

Assemble volumes using Linux device mapper

The Linux device mapper linear target can be used to assemble several disks into one device mapper device (this is what LVM uses to combine disks).

First create loop devices for the different image files:

losetup /dev/loop1 /media/backup2/urbackup/win2012server/170302-2131_Image_SYSVOL_rw/Image_SYSVOL_170302-2131.raw --offset 524288
losetup /dev/loop2 /media/backup2/urbackup/win2012server/170302-2132_Image_C_rw/Image_C_170302-2132.raw --offset 524288

Then as a final step setup the new device mapper device. The offsets and sizes are from the partition table above:

echo "0 2048 linear /dev/loop0 0
2048 716800 linear /dev/loop1 0
718848 188022784 linear /dev/loop2 0" | dmsetup create win2012server_img

The assembled disk image is then at /dev/mapper/win2012server_img.


Related content