Thursday, September 17, 2015

what's cooking in sfdisk for v2.28

I have worked on new features for sfdisk during v2.27 stabilisation. The most user visible change is partitions resize/move improvements. Now (in git tree) sfdisk is able to move partition together with data on the partition. Let's imagine you have disk with three partitions:
Device       Start     End Sectors  Size Type
/dev/sdc1     2048  206847  204800  100M Linux filesystem
/dev/sdc2   206848 1230847 1024000  500M Linux filesystem
/dev/sdc3  1230848 2047966  817119  399M Linux filesystem
where the last partition is your ex-girlfriend /home and the first partition is almost full of pictures. Now you want to remove the last unnecessary partition and enlarge the first partition. And you don't want to lost data on the first and second partitions.

(You have backup of all data, because you understand that all operations related to the partition table are risky. Right? :-)

Let's delete the last partition:

  # sfdisk /dev/sdc --delete 3
Now move the second partition to the end of the device. The size of the deleted sdc3 was 817119 sectors. The sfdisk expects input in format
  [+]start[,[+]size[,type]]
where "+num" means offset or size relative to the original partition setting. If any number is unspecified than default is to use the current setting -- it means that "+817119" is enough. (Note that -N 2 means second partition.)
# echo '+817119' | sfdisk /dev/sdc -N 2 --move-data
...
Old situation:
 
Device      Start     End Sectors  Size Type
/dev/sdc1    2048  206847  204800  100M Linux filesystem
/dev/sdc2  206848 1230847 1024000  500M Linux filesystem
 
New situation:                        

Device       Start     End Sectors  Size Type            
/dev/sdc1     2048  206847  204800  100M Linux filesystem
/dev/sdc2  1023967 2047966 1024000  500M Linux filesystem

Data move:                            
 typescript file: /root/sfdisk-sdc2.move
 old start: 206848, new start: 1023967 (move 1024000 sectors)
The important detail is --move-data option, it forces sfdisk after partition table modification copy data from old area to the new offsets. The source and target may overlap (sfdisk copy sectors in backward order if necessary). The file /root/sfdisk-sdc2.move is log with details about the change.

The last step is to enlarge the first partition to use all available free space (space originally used by the second partition). The string ",+" keeps start offset unchanged and the size is specified as "+" (all available space).

# echo ',+' | sfdisk /dev/sdc -N 1
...
Old situation:
 
Device       Start     End Sectors  Size Type
/dev/sdc1     2048  206847  204800  100M Linux filesystem
/dev/sdc2  1023967 2047966 1024000  500M Linux filesystem
 
New situation:
 
Device       Start     End Sectors  Size Type
/dev/sdc1     2048 1023966 1021919  499M Linux filesystem
/dev/sdc2  1023967 2047966 1024000  500M Linux filesystem
And now inform filesystem about a new space:
# resize2fs /dev/sdc1
and test all by mount:
# mount /dev/sdc1 /mnt/A
# mount /dev/sdc2 /mnt/B
 
# lsblk /dev/sdc
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sdc      8:32   0 1000M  0 disk
├─sdc1   8:33   0  499M  0 part /mnt/A
└─sdc2   8:34   0  500M  0 part /mnt/B

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