Re: [Qemu-devel] [PATCH v2 1/2] qapi: Introduce blockdev-group-snapshot-sync command

[Kevin Wolf]

Am 27.02.2012 18:02, schrieb Jeff Cody:
>>> +
>>> +    /* keep the same entry in bdrv_states */
>>> +    pstrcpy(tmp.device_name, sizeof(tmp.device_name), bs_top->device_name);
>>> +    tmp.list = bs_top->list;
>>> +
>>> +    /* swap contents of the fixed new bs and the current top */
>>> +    *bs_new = *bs_top;
>>> +    *bs_top = tmp;
>>> +
>>> +    bdrv_detach_dev(bs_new, bs_new->dev);
>>> +}
>>
>> The last line would actually deserve a comment /* clear the copied
>> fields in the new backing file */, which makes clear that there are
>> probably some more fields missing in this section.
> 
> OK, added.

Only the comment or also clearing other fields? For some of them it's
very obvious that they need to be cleared (copy on read, I/O throttling).

>>> +    }
>>> +
>>> +    /*
>>> +     * Now we will drain, flush, & pivot everything - we are committed at 
>>> this
>>> +     * point.
>>> +     */
>>> +    bdrv_drain_all();
>>
>> I would feel more comfortable if we could do the bdrv_drain_all() at the
>> very beginning of the function. Not that I know of a specific scenario
>> that would go wrong, but you have a nested main loop here that could do
>> more or less anything.
> 
> I can move it up to the beginning if desired...  My thought was that it
> was best to drain closer to the point of commit.

As long as we don't create new AIO requests here, drained is drained.

But anyway, I'm not requesting a change here, it was just a feeling.

>>
>>> +    QSIMPLEQ_FOREACH(states, &snap_bdrv_states, entry) {
>>> +        bdrv_flush(states->old_bs);
>>
>> This can return an error which must be checked. And of course, we must
>> do it before committing to the snapshot (but after bdrv_drain_all).
> 
> Hmm. If the flush returns error, do we abandon at this point? Perhaps it
> would be best to loop through and flush each device first, and if no
> flushes fail, _then_ loop through and perform the bdrv_append(). Once we
> are calling bdrv_append(), we want no possible failure points.

Yes, this is what I meant. Sorry for the somewhat vague wording.

Kevin