[ovirt-users] Storage migration: Preallocation forced on destination

[Nir Soffer]

On Fri, Mar 4, 2016 at 7:07 PM, Nicolás <nicolas at devels.es> wrote:
> Hi,
>
> We're migrating an existing storage (glusterfs) to a new one (iSCSI). All
> disks on glusterfs are thin provisioned, but when migrating to iSCSI the
> following warning is shown:
>
>     The following disks will become preallocated, and may consume
> considerably more space on the target: local-disk
>
> Why is that? Is there a way to migrate disks so they are thin provisioned on
> iSCSI as well?

The issue is that we use raw sparse format for thin provisioned disks
on file based
storage. The file system provides the thin provisioning, maintaining holes in
the files.

When we create the destination lv, we use the disk virtual size, so you get
practically a preallocated volume.

I think we can do better - before copying the disk, we can check the actual used
space (e.g. what qemu-img info or stat report), and create the
destination lv using
the used size (plus additional space for qcow format).

I tested this by extending the destination lv manually and then
copying data manually
using qemu-img convert, and it works.

Can you file a bug for this, and explain the use case?

Nir