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[Qemu-discuss] qemu-img convert to thin LV
From: |
Edoardo Comar |
Subject: |
[Qemu-discuss] qemu-img convert to thin LV |
Date: |
Fri, 28 Jun 2013 10:25:51 +0100 |
Hi,
Is there a way to convert an image to an LV without having to allocate the
full disk capacity ?
Is qemu-img convert compatible with thin-provisioned LVs as targets ?
I wanted to convert a file-based image to a LV image.
The file-based image has a capacity much larger than the actual data it
contains
so it has a small footprint on disk (either a qcow2 or a sparse file raw
image).
If I use qemu-img convert -O raw ... with a target LV *sized to the full
capacity of the file-image* all is fine.
However, if I try use an LV whose virtual size equals the capacity, but
its size is smaller (although more than enough for the data)
using the capability as described in the lvcreate manpage :
"lvcreate --virtualsize 1T --size 100M --snapshot --name sparse vg1"
creates a sparse device named /dev/vg1/sparse of size 1TB with
space
for just under 100MB of actual data on it.
then after the qemu-img convert passes without errors, the resulting LV is
invariably corrupt (unmountable).
If alternatively, I try use a thin-provisioned LV pool, and a LV backed by
that pool,
as described in
https://access.redhat.com/site/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Logical_Volume_Manager_Administration/thinly_provisioned_volume_creation.html
the LV resulting from qemu-img convert is not corrupt, but it uses as much
space as the capacity of file-image it came from.
This on both RHEL64 and Fedora18.
Is there a way to convert an image to an LV without having to allocate the
full disk capacity ?
The rationale is that if the LV is then only accessed via snapshots, the
backing LV can remain small while the snapshot can grow,
and overall I can save disk space (rather than using 2x image capacity).
regards,
Edoardo Comar
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