[Sheepdog] Deleting snapshots

[Floris Bos]

Hi,

On Saturday, October 02, 2010 01:42:43 pm MORITA Kazutaka wrote:
> Floris Bos wrote:
> > Hi,
> > 
> > I'm writing an alternative sheepdog client, and am looking for a way to
> > delete a (temporary) snapshot.
> 
> Great!  What kind of client are you writing?

A simple libfuse-based client, that allows you to access the sheepdog VDIs as 
regular files in the file system.
That way you can use legacy tools like rsync to backup your virtual machines, 
e.g.:

==
root at vpser:~# collie vm list
Name            |Vdi size |Allocated| Shared  | Status
----------------+---------+---------+---------+------------
vm1             |   9.8 GB|   964 MB|   0.0 MB| not running
vm2             |   9.8 GB|   1.9 GB|   0.0 MB| not running
root at vpser:~# fusedog /mnt
root at vpser:~# ls -l /mnt
total 0
-rw------- 1 root root 10485760000 2010-10-02 17:34 vm1
-rw------- 1 root root 10485760000 2010-10-02 17:35 vm2
root at vpser:~# rsync -avzS /mnt max at name-of-my-backup-server:/home/max/vmbackup
Password:
sending incremental file list
created directory /home/max/vmbackup
mnt/
mnt/vm1
mnt/vm2
==

Would like to add automatic snapshot support so that it can create (temporary 
read-only) snapshots to backup VMs that are currently running.


> > I noticed that there is a SD_OP_DEL_VDI in include/sheep.h
> > But I cannot find a similar operation in include/sheepdog_proto.h
> > 
> > 
> > Is it safe for an external program to simply send SD_OP_DEL_VDI for
> > deleting snapshots?
> 
> Yes, it should be safe.  Indeed, SD_OP_DEL_VDI should be in
> include/sheepdog_proto.h because we need to support deleting snapshots
> from qemu-img commands in future.
> 
> See vdi_delete() in collie/collie.c.  It would tell you how to use a
> SD_OP_DEL_VDI operation.

It seems that snapshot creation and deletion works differently than the way I 
want to use them.

After creating a snapshot, it looks like the newest vdi_id becomes the new 
main vdi (as it is the only item without a "s" in the "vdi list").

==
max at vpser:~$ collie vm list
Name            |Vdi size |Allocated| Shared  | Status
----------------+---------+---------+---------+------------
test05b         |   9.8 GB|   0.0 MB|   964 MB| not running
max at vpser:~$ collie vdi list
  name        id    size    used  shared    creation time   vdi id
------------------------------------------------------------------
s test05b      1  9.8 GB  964 MB  0.0 MB 2010-09-27 18:40   662da4
s test05b      2  9.8 GB  0.0 MB  964 MB 2010-10-02 16:42   662da5
  test05b      3  9.8 GB  0.0 MB  964 MB 2010-10-02 17:03   662da6
==

And if I use SD_OP_DEL_VDI to delete the newest vid in my client, "collie vm 
list" returns empty.

==
max at vpser:~$ collie vdi list
  name        id    size    used  shared    creation time   vdi id
------------------------------------------------------------------
s test05b      1  9.8 GB  964 MB  0.0 MB 2010-09-27 18:40   662da4
s test05b      2  9.8 GB  0.0 MB  964 MB 2010-10-02 16:42   662da5
max at vpser:~$ collie vm list
Name            |Vdi size |Allocated| Shared  | Status
----------------+---------+---------+---------+------------
==


The command I use to create a snapshot looks like this:

==
	memcpy(buf, parent->name, sizeof(buf));
	struct sd_vdi_req req = {
		.proto_ver   = SD_PROTO_VER,
		.opcode      = SD_OP_NEW_VDI,
		.base_vdi_id = parent->vdi_id,
		.flags		 = SD_FLAG_CMD_WRITE,
		.data_length = sizeof(buf),
		.vdi_size	 = parent->vdi_size,
		.snapid		 = 1
	};
==

(where "parent" is the "struct sheepdog_inode" of the original VDI)

Is there any option I can use to tell sheepdog not to promote the new snapshot 
as main VDI?


-- 
Yours sincerely,

Floris Bos