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Re: [libvirt] Extensions to the libvirt Storage API
- From: Patrick Dignan <pat_dignan dell com>
- To: "Daniel P. Berrange" <berrange redhat com>
- Cc: "libvir-list redhat com" <libvir-list redhat com>
- Subject: Re: [libvirt] Extensions to the libvirt Storage API
- Date: Wed, 28 Jul 2010 12:24:53 -0500
On 07/28/2010 05:08 AM, Daniel P. Berrange wrote:
Understandable. I was thinking that there is currently no way to
specify a vendor of a storage backend. For example, an iSCSI vendor.
This makes it look like implementing vendor-specific extensions requires
creating a new backend, even though there's already an iSCSI backend.
It seems like a secondary field for vendor, and maybe even model, could
On Tue, Jul 27, 2010 at 06:28:01PM -0500, Patrick Dignan wrote:
I'm looking at implementing some functionality in libvirt that would
allow it to call functions in an unpublished iSCSI library. Some of the
functionality I wish to implement is not currently part of the libvirt
storage API. I wanted to suggest the following additions to the storage
API: grow volumes, show whether thin provisioning is enabled, enable
thin provisioning, disable thin provisioning, create snapshots, and
delete snapshots. I've added a patch at the end of the mail showing how
I think these functions should be implemented. Note that I have not
included details about the virStorageSnapshotDefPtr yet, that's the next
Perhaps this should be in a separate mail for better threading, but it
seems a bit strange to me that the storage interface isn't pluggable in
the traditional sense. In order to add a backend to libvirt, one has to
make modifications all over the place, for example: virt-inst, the
Makefile.am, the configure.ac, storage_backend.h, and several other
places. It would make sense to me to make this pluggable such that
someone could just load in a library that implements the required
functions and some identifying information (eg type of storage,
description, etc). A list of supported backends could be stored in
empty files in a directory somewhere, or some similar hack. This way
someone could write a plugin for tgtd for example, or in my case the
library I'm working with. I think this would also help others with
writing plugins for more storage backends. How difficult do you think
this would be? I'm willing to do a reasonable amount of work to get
this implemented, but I want to know what the experts think!
We explicitly don't support external driver plugins in libvirt for a
couple of reasons
- We don't want to support use of closed source plugins
- We don't want to guarentee stability of any aspect of
libvirt's internal API
We would like to see support for the various vendor specific iSCSI
extensions to allow volume creation/deletion, but want that code to
be part of the libvirt codebase.
Do all backends support shrinking? I was under the impression shrinking
is not quite a universal feature, so it made sense to me to break this
out. If most backends support shrinking, it makes sense to use
resizeVol instead then.
/* File creation/cloning functions used for cloning between backends */
int virStorageBackendCreateRaw(virConnectPtr conn,
@@ -76,6 +83,12 @@
+ virStorageBackendGrowVol growVol;
I'd call this 'resizeVol' since there's no reason we can't also support
Sorry, that was a bit of a misnomer on my part, these functions are
intended to be used at the volume level. However, what if they want to
enable thin provisioning or disable it after the fact? Is that not
going to be supported? An example use case of enabling after the fact
is if they want to enable it, and then grow the volume to a larger
size. Disabling, of course, could be done at any time (eg for a speed
increase). Do you object to the thinProvisionShow function, in either case?
+ virStorageBackendThinProvisionShow thinProvisionShow;
+ virStorageBackendThinProvisionEnable thinProvisionEnable;
+ virStorageBackendThinProvisionDisable thinProvisionDisable;
I'm not really liking this as a concept. The other storage drivers, and
indeed my iSCSI server, deal with thin provisioning on a per-volume basis
when creating the volume. The libvirt model is that if in the XML, then
<allocation> value is zero then the user is requesting thin provisioning
of that volume. ie no storage allocated for it upfront. If<allocation>
matches<capacity> then the volume should be fully allocated upfront.
I wasn't aware of this functionality. It looks like it's implemented on
a per-hypervisor basis. It'd be really cool to get snapshotting
integrated into storage backends with snapshotting support, so that
snapshots would show up in both libvirt and the storage backend's UI,
but I can see how this would be nearly impossible.
+ virStorageBackendCreateSnapshot createSnapshot;
+ virStorageBackendDeleteSnapshot deleteSnapshot;
There's no need for snapshot APIs. This functionality is already supported
via the normal volume creation API, just specify the original volume to be
snapshotted in the XML as the backing store.
Thanks for the quick reply!
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