How create thin lvm volume from sparse disk image?

flemmi2k · 2019-02-25 17:00:46

Hi all,
I would like to move disk images of some virtual machines to thinly provisioned lvm volumes.
The VM disk image is in a sparse raw file:

root@penny smb # ls -lksh vm-win10.raw
24G -rw-r--r-- 1 root root 80G Feb 24 18:45 vm-win10.raw

It takes 24G to store the 80G disk.

I created a thin logical volume as described in lvmthin(7) man page:

root@penny smb # lvcreate -n vm-win -V 85899345920b --thinpool pool0 vg0
  Logical volume "vm-win" create

Then, I dd'd the disk to the logical volume

root@penny smb # dd if=vm-win10.raw of=/dev/vg0/vm-win bs=1k status=progress
85831631872 bytes (86 GB, 80 GiB) copied, 2022 s, 42.4 MB/s
83886080+0 records in
83886080+0 records out
85899345920 bytes (86 GB, 80 GiB) copied, 2023.69 s, 42.4 MB/s

It seems zero chunks were written to vm-win as well and the size of the logical volume is 80G and not 24G

root@penny smb # lvs
  LV     VG  Attr       LSize   Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
  arch   vg0 -wi-ao---- 300.00g                                                     
  pool0  vg0 twi-aotz-- 300.00g              35.67  7.03                            
  vm-win vg0 Vwi-a-tz--  80.00g pool0        100.00                                 
  work   vg0 Vwi-aotz-- 100.00g pool0        27.01

What went wrong? I know there are other ways to store VM disks, but I'd like to learn more about thin provisioning.
Thanks

frostschutz · 2019-02-25 17:05:37

by default dd copies everything, zero or not, dd does not care

but then, dd has conv=sparse to skip zero

use it with caution (only reliable if the target will actually return zero for the skipped areas - as there may be deliberate non-free zero)

other option would be to run fstrim after the copy, or copy files instead of raw

you can also query extents with filefrag then copy each extent individually, but that requires even more care to get all the correct segments at the correct offsets

Last edited by frostschutz (2019-02-25 17:06:37)

flemmi2k · 2019-02-25 17:56:31

conv=sparse did the trick

root@penny smb # dd if=vm-win10.raw of=/dev/vg0/vm-win conv=sparse bs=4k status=progress
83788333056 bytes (84 GB, 78 GiB) copied, 880 s, 95.2 MB/s
20971520+0 records in
20971520+0 records out
85899345920 bytes (86 GB, 80 GiB) copied, 881.115 s, 97.5 MB/s
root@penny smb # lvs
  LV     VG  Attr       LSize   Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
  arch   vg0 -wi-ao---- 300.00g                                                     
  pool0  vg0 twi-aotz-- 300.00g              17.31  4.28                            
  vm-win vg0 Vwi-a-tz--  80.00g pool0        31.16                                  
  work   vg0 Vwi-aotz-- 100.00g pool0        27.01

Arch Linux

#1 2019-02-25 17:00:46

How create thin lvm volume from sparse disk image?

#2 2019-02-25 17:05:37

Re: How create thin lvm volume from sparse disk image?

#3 2019-02-25 17:56:31

Re: How create thin lvm volume from sparse disk image?

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