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.
