[Sheepdog] Deleting snapshots

[MORITA Kazutaka]

At Sat, 2 Oct 2010 18:07:18 +0200,
Floris Bos wrote:
> 
> 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, 

Sounds good!  It would also be helpful for debugging a sheepdog
server.

> 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").

Yes.  Sheepdog makes the current vdi read-only (snapshot vdi), and
creates a new vdi as a writable one (main vdi).

> 
> ==
> 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.

It seems that deleting the latest (non-snapshot) vdi doesn't work
properly.  'collie vm list' should show the test05b vdi in this case.

> 
> ==
> 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?
> 

Currently, no.  However, I think it would be better to reserve 0 as a
snap_id of the latest vdi (main vdi).  In this case, taking snapshots
doesn't change a snapshot id of the latest vdi.  The id is fixed to
zero always.  Is this the behavior you want?


Thanks,

Kazutaka