Re: [Qemu-devel] [sheepdog] [PATCH 2/2] sheepdog: refine discard support

[Hitoshi Mitake]

On Fri, Sep 4, 2015 at 5:51 PM, Hitoshi Mitake <address@hidden> wrote:
> On Wed, Sep 2, 2015 at 9:36 PM, Vasiliy Tolstov <address@hidden> wrote:
>> 2015-09-01 6:03 GMT+03:00 Hitoshi Mitake <address@hidden>:
>>> This patch refines discard support of the sheepdog driver. The
>>> existing discard mechanism was implemented on SD_OP_DISCARD_OBJ, which
>>> was introduced before fine grained reference counting on newer
>>> sheepdog. It doesn't care about relations of snapshots and clones and
>>> discards objects unconditionally.
>>>
>>> With this patch, the driver just updates an inode object for updating
>>> reference. Removing the object is done in sheep process side.
>>>
>>> Cc: Teruaki Ishizaki <address@hidden>
>>> Cc: Vasiliy Tolstov <address@hidden>
>>> Cc: Jeff Cody <address@hidden>
>>> Signed-off-by: Hitoshi Mitake <address@hidden>
>>
>>
>> I'm test this patch and now discard working properly and no errors in
>> sheepdog log file.
>>
>> Tested-by: Vasiliy Tolstov <address@hidden>
>
> On the second thought, this patch has a problem of handling snapshot.
> Please drop this one (1st patch is ok to apply).
>
> I'll solve the problem in sheepdog side.

On the third thought, this patch can work well ;) Please pick this patch, Jeff.

I considered about a case of interleaving of snapshotting and
discarding requests like below:
1. user invokes dog vdi snapshot, a command for making current VDI
snapshot (updating inode objects)
2. discard request from VM before the request of 1 completes
3. discard completes
4. request of 1 completes

In this case, some data_vdi_id of original inode can be overwritten
before completion of snapshotting. However, this behavior is valid
because dog vdi snapshot doesn't return ack to the user.

Thanks,
Hitoshi

>
> Thanks,
> Hitoshi
>
>>
>> --
>> Vasiliy Tolstov,
>> e-mail: address@hidden
>> --
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