[libvirt] [PATCH v2] qemu-migration: Disallow migration of read only disk

[Peter Krempa]

On Tue, Sep 20, 2016 at 09:40:22 -0400, Corey S. McQuay wrote:
> Currently Libvirt allows attempts to migrate read only disks. Qemu cannot handle this as read only
> disks cannot be written to on the destination system. The end result is a cryptic error message
> and a failed migration.

This is not necessarily true. Read only disks can sometimes be in fact
backed by storage that is writable and it's desired to be migrated.

I think you want to specify which storage to migrate using the
VIR_MIGRATE_PARAM_MIGRATE_DISKS migration parameter for
virDomainMigrate3.

http://libvirt.org/html/libvirt-libvirt-domain.html#virDomainMigrate3

http://libvirt.org/html/libvirt-libvirt-domain.html#VIR_MIGRATE_PARAM_MIGRATE_DISKS

Virsh also implements support for that flag.

> This patch causes migration to fail earlier and provides a meaningful error message stating that
> migrating read only disks is not supported.
> 
> Signed-off-by: Corey S. McQuay <csmcquay at linux.vnet.ibm.com>
> Reviewed-by: Jason J. Herne <jjherne at linux.vnet.ibm.com>
> Reviewed-by: Boris Fiuczynski <fiuczy at linux.vnet.ibm.com>
> ---
>  src/qemu/qemu_migration.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/src/qemu/qemu_migration.c b/src/qemu/qemu_migration.c
> index e451ef6..011f349 100644
> --- a/src/qemu/qemu_migration.c
> +++ b/src/qemu/qemu_migration.c
> @@ -2157,6 +2157,13 @@ qemuMigrationDriveMirror(virQEMUDriverPtr driver,
>          if (!qemuMigrateDisk(disk, nmigrate_disks, migrate_disks))
>              continue;
>  
> +        if (disk->src->readonly) {
> +            virReportError(VIR_ERR_OPERATION_UNSUPPORTED,
> +                       _("Cannot migrate read-only disk %s"),
> +                       disk->dst);
> +            goto cleanup;
> +        }

NACK this would break the above use case. You want to specify explicitly
which storage to migrate using the above described API.

Peter