[Sheepdog] [PATCH v3] sheep: change snapshot/clone flow

[MORITA Kazutaka]

At Mon, 23 Apr 2012 11:37:11 +0800,
HaiTing Yao wrote:
> 
> On Sat, Apr 21, 2012 at 4:56 AM, MORITA Kazutaka
> <morita.kazutaka at gmail.com>wrote:
> 
> > At Fri, 20 Apr 2012 14:40:26 +0800,
> >  yaohaiting.wujue at gmail.com wrote:
> > >
> > > From: HaiTing Yao <wujue.yht at taobao.com>
> > >
> > > When create snapshot for source VDI, the new created VDI used as source
> > > VDI, and the old source VDI used as snapshot. This flow make users
> > > confused about VDI and snapshot relation. The snapshot metadata maybe is
> > > stored on multi-VDI, so need read multi VDIs inode to get snapshot list.
> > >
> > > When create snapshot, we does not need change new created VDI to source
> > > VDI. The source VDI just need use snapshot VDI ID as its object data ID.
> > >
> > > Show one example.
> > >
> > > Before modification:
> > >
> > >   Name        Id    Size    Used  Shared    Creation time   VDI id   Tag
> > > s v1           1   64 MB   20 MB  0.0 MB 2012-03-26 16:55   709128
> > > s v1           2   64 MB  0.0 MB   20 MB 2012-03-26 16:56   709129   sn3
> > >   v1           3   64 MB  0.0 MB   20 MB 2012-03-26 16:56   70912a
> > >
> > > After modification:
> > >
> > >   Name        Id    Size    Used  Shared    Creation time   VDI id   Tag
> > >   v1           0   64 MB   20 MB  0.0 MB 2012-03-27 11:06   709128
> > > s v1           1   64 MB  0.0 MB   20 MB 2012-03-27 11:06   709129
> > > s v1           2   64 MB  0.0 MB   20 MB 2012-03-27 11:07   70912a   sn3
> > >
> > > Signed-off-by: HaiTing Yao <wujue.yht at taobao.com>
> > > ---
> > >  collie/common.c          |    2 +-
> > >  collie/vdi.c             |   33 ++++++++++++++++++++++-----------
> > >  include/sheepdog_proto.h |    6 ++++--
> > >  sheep/vdi.c              |   18 ++++++++++--------
> > >  4 files changed, 37 insertions(+), 22 deletions(-)
> >
> > Your patch breaks the relationship between VDI snapshots.
> > For example:
> > ==
> >  $ collie cluster format -c 1
> >
> >  $ qemu-img create sheepdog:test 4G
> >  $ qemu-img snapshot -c tag1 sheepdog:test
> >  $ qemu-img snapshot -c tag2 sheepdog:test
> >  $ qemu-img snapshot -c tag3 sheepdog:test
> >
> >  $ qemu-img create sheepdog:test2 4G
> >  $ qemu-img snapshot -c tag1 sheepdog:test2
> >  $ qemu-img snapshot -c tag2 sheepdog:test2
> >  $ qemu-io -c "write 0 512" sheepdog:test2:1
> >  $ qemu-img snapshot -c tag3 sheepdog:test2
> >
> >  $ collie vdi tree
> > ==
> >
> > and the expected output is:
> > ==
> >  test---[2012-04-21 05:51]---[2012-04-21 05:51]---[2012-04-21
> > 05:51]---(you are here)
> >  test2---[2012-04-21 05:51]-+-[2012-04-21 05:51]---[2012-04-21 05:51]
> >                             `-[2012-04-21 05:51]---(you are here)
> > ==
> >
> 
> I think you have applied my block/sheepdog.c patch for qemu project. There
> is still something to do. qemu-img is still having some problem with object
> cache when there is snapshot. The work is on the progress by Liuyuan as I
> knew. Perhaps you can shutdown object cache temporarily by this:

No, the cache is not a problem here.


> 
> void do_io_request(struct work *work)
> 
>         dprintf("%x, %" PRIx64" , %u\n", opcode, oid, epoch);
>        + hdr->flags = hdr->flags & ~SD_FLAG_CMD_CACHE;
>         if (hdr->flags & SD_FLAG_CMD_RECOVERY)
> 
> Then the display like this
> 
> [wujue.yht at v134092.sqa.cm4 ~]$ clv
>   Name        Id    Size    Used  Shared    Creation time   VDI id  Tag
>   v1           0  1.0 GB  0.0 MB  0.0 MB 2012-04-23 10:45   709128
>   img1         0  1.0 GB  0.0 MB  0.0 MB 2012-04-23 10:46   9e028f
> s img1         1  1.0 GB  0.0 MB  0.0 MB 2012-04-23 10:46   9e0290  img1-sn1
> s img1         2  1.0 GB  0.0 MB  0.0 MB 2012-04-23 10:47   9e0291  img1-sn2
> s img1         3  1.0 GB  0.0 MB  0.0 MB 2012-04-23 10:47   9e0292  img1-sn3
>   img2         0  1.0 GB  0.0 MB  0.0 MB 2012-04-23 10:47   9e0442
> [wujue.yht at v134092.sqa.cm4 ~]$ ./collie vdi tree
> v1---(you are here)
> img1---(you are here)-+-[2012-04-23 10:46]
>                       |-[2012-04-23 10:47]
>                       `-[2012-04-23 10:47]
> img2---(you are here)
> The snapshots will not display on one line, because all of snapshots belong
> to base VDI now and there is no direct relation between snapshots. I am not
> sure of this. Do we need display them on one line?
> 
> Because the base VDI never be changed, the postion identified by 'you are
> here' always follows base VDI after my change. Do we expect this?

The problem is that your patch doesn't set VDI relationships
correctly.  'parent_vdi_id' in struct sheepdog_inode should be the
source vdi id, and 'child_vdi_id' should contain children vdi.  If you
create a new vdi (or snapshot) B from A, A is a parent of B.


> BTW, since we change the inode structure, how about increse the
> SD_PROTO_VER?

If we would merge this change, we must increment SD_PROTO_VER.  But
I'm still not sure this change is really necessary.  As I said before,
I'd like to avoid changing the sheepdog_inode structure if possible.
If we don't show VDI id in the output of 'collie vdi list', there is
no confusing issue you want to fix, no?

Thanks,

Kazutaka