[Qemu-discuss] Converting qcow2 image to raw thin lv

[Nir Soffer]

Hi all,

I'm trying to convert images (mostly qcow2) to raw format on thin lv,
hoping to write only the allocated blocks on the thin lv, but
it seems that qemu-img cannot write sparse image on a block
device.

Here is an example:

Create a new thin lv:

# lvcreate --name raw-test --virtualsize 20g --thinpool pool0 ovirt-local
  Using default stripesize 64.00 KiB.
  Logical volume "raw-test" created.

address@hidden ~]# lvs ovirt-local
  LV                                   VG          Attr       LSize
Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
  029060ab-41ef-4dfd-9a3e-4c716c01db06 ovirt-local Vwi-a-tz-- 20.00g
pool0        6.74
  4f207ee8-bb47-465a-9b68-cb778e070861 ovirt-local Vwi-a-tz-- 20.00g
pool0        0.00
  7aed605e-c74c-40d8-b449-8a1bf7228b8b ovirt-local Vwi-a-tz-- 20.00g
pool0        6.98
  ce6d08d3-350f-4afa-a0e7-7b492a1a7744 ovirt-local Vwi-a-tz-- 20.00g
pool0        6.87
  pool0                                ovirt-local twi-aotz-- 40.00g
           10.30  5.49
  raw-test                             ovirt-local Vwi-a-tz-- 20.00g
pool0        0.00

I want to convert this image (fresh fedora 25 installation):

# qemu-img info fedora.qcow2
image: fedora.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 1.3G
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false

Convert the image to raw, into the new thin lv:

# qemu-img convert -p -f qcow2 -O raw -t none -T none fedora.qcow2
/dev/ovirt-local/raw-test
    (100.00/100%)

The image size was 1.3G, but now the thin lv is fully allocated:

# lvs ovirt-local
  LV                                   VG          Attr       LSize
Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
  029060ab-41ef-4dfd-9a3e-4c716c01db06 ovirt-local Vwi-a-tz-- 20.00g
pool0        6.74
  4f207ee8-bb47-465a-9b68-cb778e070861 ovirt-local Vwi-a-tz-- 20.00g
pool0        0.00
  7aed605e-c74c-40d8-b449-8a1bf7228b8b ovirt-local Vwi-a-tz-- 20.00g
pool0        6.98
  ce6d08d3-350f-4afa-a0e7-7b492a1a7744 ovirt-local Vwi-a-tz-- 20.00g
pool0        6.87
  pool0                                ovirt-local twi-aotz-- 40.00g
           60.30  29.72
  raw-test                             ovirt-local Vwi-a-tz-- 20.00g
pool0        100.00

Recreate the lv:

# lvremove -f ovirt-local/raw-test
  Logical volume "raw-test" successfully removed

# lvcreate --name raw-test --virtualsize 20g --thinpool pool0 ovirt-local
  Using default stripesize 64.00 KiB.
  Logical volume "raw-test" created.

Covert the qcow image to raw sparse file:

# qemu-img convert -p -f qcow2 -O raw -t none -T none fedora.qcow2 fedora.raw
    (100.00/100%)

# qemu-img info fedora.raw
image: fedora.raw
file format: raw
virtual size: 20G (21474836480 bytes)
disk size: 1.3G

Write the sparse file to the thin lv:

# dd if=fedora.raw of=/dev/ovirt-local/raw-test bs=8M conv=sparse
2560+0 records in
2560+0 records out
21474836480 bytes (21 GB) copied, 39.0065 s, 551 MB/s

Now we are using only 7.19% of the lv:

# lvs ovirt-local
  LV                                   VG          Attr       LSize
Pool  Origin Data%  Meta%  Move Log Cpy%Sync Convert
  029060ab-41ef-4dfd-9a3e-4c716c01db06 ovirt-local Vwi-a-tz-- 20.00g
pool0        6.74
  4f207ee8-bb47-465a-9b68-cb778e070861 ovirt-local Vwi-a-tz-- 20.00g
pool0        0.00
  7aed605e-c74c-40d8-b449-8a1bf7228b8b ovirt-local Vwi-a-tz-- 20.00g
pool0        6.98
  ce6d08d3-350f-4afa-a0e7-7b492a1a7744 ovirt-local Vwi-a-tz-- 20.00g
pool0        6.87
  pool0                                ovirt-local twi-aotz-- 40.00g
           13.89  7.17
  raw-test                             ovirt-local Vwi-a-tz-- 20.00g
pool0        7.19

This works, but it would be nicer to have a way to convert
to raw sparse to a block device in one pass.

Nir