[sheepdog] [PATCH v4] sheepdog: fix loadvm operation

[MORITA Kazutaka]

At Thu, 25 Apr 2013 20:49:39 +0800,
Liu Yuan wrote:
> 
> From: Liu Yuan <tailai.ly at taobao.com>
> 
> Currently the 'loadvm' opertaion works as following:
> 1. switch to the snapshot
> 2. mark current working VDI as a snapshot
> 3. rely on sd_create_branch to create a new working VDI based on the snapshot
> 
> This works not the same as other format as QCOW2. For e.g,
> 
> qemu > savevm # get a live snapshot snap1
> qemu > savevm # snap2
> qemu > loadvm 1 # This will steally create snap3 of the working VDI
> 
> Which will result in following snapshot chain:
> 
> base <-- snap1 <-- snap2 <-- snap3
>           ^
>           |
>       working VDI
> 
> snap3 was unnecessarily created and might be annoying users.
> 
> This patch discard the unnecessary 'snap3' creation. and implement
> rollback(loadvm) operation to the specified snapshot by
> 1. switch to the snapshot
> 2. delete working VDI
> 3. rely on sd_create_branch to create a new working VDI based on the snapshot
> 
> The snapshot chain for above example will be:
> 
> base <-- snap1 <-- snap2
>           ^
>           |
>       working VDI
> 
> Cc: qemu-devel at nongnu.org
> Cc: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>
> Cc: Kevin Wolf <kwolf at redhat.com>
> Cc: Stefan Hajnoczi <stefanha at redhat.com>
> Signed-off-by: Liu Yuan <tailai.ly at taobao.com>
> ---
> v4:
>  - print an error message when NO_VDI found
> 
> v3:
>  - let boot from snapshot behave like 'loadvm'
> 
> v2:
>  - use do_req() because sd_delete isn't in coroutine
>  - don't break old behavior if we boot up on the snapshot by using s->reverted
>    to indicate if we delete working VDI successfully
>  - fix a subtle case that sd_create_branch() isn't called yet while another
>    'loadvm' is executed
> 
>  block/sheepdog.c |   54 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 53 insertions(+), 1 deletion(-)

Reviewed-by: MORITA Kazutaka <morita.kazutaka at lab.ntt.co.jp>