Re: [Qemu-devel] [PATCH 1/2] Make cow_co_is_allocated and cow_update_bitmap more efficient

[Paolo Bonzini]

Il 21/08/2013 12:31, Charlie Shepherd ha scritto:
> On 21/08/2013 10:19, Paolo Bonzini wrote:
>> Il 21/08/2013 11:11, Charlie Shepherd ha scritto:
>>> It still seems
>>> worthwhile to me to be as efficient as possible, I guess that means
>>> processing a sector's worth of metadata at a time?
>> Yes, that's what my patches do.  My is_allocated and flushing strategy +
>> something like your replacement of cow_set_bit (just without the
>> unbounded allocation) should be pretty good.
>>
>> Perhaps you can use a cow_co_is_allocated loop after writing the data.
>> If it returns 0, you flush (the first time only) and call your
>> cow_update_bitmap.  Then you advance by num_same sectors and go on until
>> you did all the nb_sectors.  The disadvantage is that it does two reads
>> (one in cow_co_is_allocated, one in cow_update_bitmap).  The advantage
>> is that unbounded allocation goes away because cow_co_is_allocated will
>> never consider more than a sector of bitmap data.  And you can reuse all
>> your cow_update_bitmap code.
> 
> Agreed. But can the two functions not share the same read data?

Yes, of course!  Good idea.

Paolo