On Mon, Oct 29, 2018 at 13:35:36 +0300, Nikolay Shirokovskiy wrote:

External snapshot of readonly disk of active domain is impossible but error message [1] is cryptic (it's source is qemu). Let's make error message more user friendly.

The problem is libvirtd precreates snapshot image with no write permission which is not expected by qemu.

[1] current error message error: internal error: unable to execute QEMU command 'transaction': Could not create file: Permission denied

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com> ---

By the way if domain is not active then snapshot is possible. However top image will have write permissions after snapshot. We can make snapshot work for active domain I guess if we precreate writable snapshot image, but then we end up running readonly disk on writable image. Also making snapshot of readonly disk is not that practical so let's just fix error message for now.

We should not allow doing this at all then. Not even when inactive.

src/qemu/qemu_driver.c | 8 ++++++++ 1 file changed, 8 insertions(+)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c index a52e249..e75931e 100644 --- a/src/qemu/qemu_driver.c +++ b/src/qemu/qemu_driver.c @@ -14683,6 +14683,14 @@ qemuDomainSnapshotPrepare(virDomainObjPtr vm, active, reuse) < 0) goto cleanup;

+ if (dom_disk->src->readonly && active) { + virReportError(VIR_ERR_CONFIG_UNSUPPORTED, + _("external snapshot for readonly disk %s " + "of active domain is not supported"), + disk->name); + goto cleanup; + }

Please put this into qemuDomainSnapshotPrepareDiskExternal. qemuDomainSnapshotPrepare is already too long and we don't need to make it even longer. Also as noted I don't think we should allow this even when inactive.