Re: [Qemu-devel] Re: [PATCH v2 6/7] qed: Read/write support

[Avi Kivity <avi@redhat.com>]

  On 10/13/2010 04:53 PM, Anthony Liguori wrote:
> On 10/13/2010 09:16 AM, Avi Kivity wrote:
>>  On 10/13/2010 04:11 PM, Anthony Liguori wrote:
>>>> Why would you ever update the header, apart from relocating L1 for 
>>>> some reason?
>>>
>>>
>>> To update the L1/L2 tables clean bit.  That's what prevents a check 
>>> in the normal case where you have a clean shutdown.
>>
>> I see - so you wouldn't update it every allocation, only when the 
>> disk has been quiet for a while.
>
> Right, the current plan is to flush the header dirty bit on shutdown 
> or whenever there is an explicit flush of the device.  Current that is 
> caused by either a guest-initiated flush or a L1 update.

That does add an extra write (and a new write+flush later to mark the 
header dirty again when you start allocating).  I'd drop it and only use 
the timer.

in fact, it adds an extra flush too.  The sequence

1 L1 update
2 mark clean
3 flush

is unsafe since you can crash between 2 and 3, ad only 2 makes it.  So 
I'd do something like

1 opportunistic flush (for whatever reason)
2 set timer
3 no intervening metadata changes
4 mark clean
5 no intervening metadata changes
6 mark dirty
7 flush
8 metadata changes


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.