[dm-devel] DM_snapshot_cow filesystem (dmsetup create snapshot)

[Alasdair G Kergon]

On Thu, Dec 01, 2011 at 09:44:53PM -0600, Douglas McClendon wrote:
> The commandline to do the 'edit' was requested for users like Fred, and  
> because I guess one day I might find myself wanting to do the same.

This is unsupported, but if you want to try:
You need to flip the 5th byte of the 'cow' device from 0 to 1 (when
the snapshot is not active).  (But you might have lost the most recent
changes if they weren't flushed out when it filled up.)

Something like this:

# lvs
  LV    VG   Attr   LSize  Origin Snap%  Move Log Copy%  Convert
  lvol0 vg2  owi-a- 10.00m
  lvol1 vg2  Swi-I- 10.00m lvol0  100.00

# dmsetup table vg2-lvol1-cow
0 20480 linear 7:0 22528

# lvchange -an vg2/lvol0

# dmsetup create tmp-vg2-lvol1-cow --table "0 20480 linear 7:0 22528"

# hexdump -C /dev/mapper/tmp-vg2-lvol1-cow | head -1
00000000  53 6e 41 70 00 00 00 00  01 00 00 00 08 00 00 00  |SnAp............|

Flip byte 5, e.g. copy to file, edit that bit, copy back.
  # dd if=/dev/mapper/tmp-vg2-lvol1-cow of=/tmp/cow-start bs=512 count=1
  # vim -b /tmp/cow-start   (and/or xxd)
  # dd if=/tmp/cow-start of=/dev/mapper/tmp-vg2-lvol1-cow  

# dmsetup remove tmp-vg2-lvol1-cow

# lvchange -ay vg2/lvol0

# lvs
  LV    VG   Attr   LSize  Origin Snap%  Move Log Copy%  Convert
  lvol0 vg2  owi-a- 10.00m                                      
  lvol1 vg2  swi-a- 10.00m lvol0   10.08                        

# hexdump -C /dev/mapper/vg2-lvol1-cow | head -1
00000000  53 6e 41 70 01 00 00 00  01 00 00 00 08 00 00 00  |SnAp............|

Alasdair