Il 21/08/2013 11:11, Charlie Shepherd ha scritto: >>> >> For cow_co_is_allocated, you have the luxury of returning information >> only for the fewer than nb_sectors. That is, you can set *num_same to a >> smaller value than nb_sectors, even if sector_num + *num_same has the >> same state as the [sector_num, sector_num + *num_same) range. It will >> cause extra calls to is_allocated in the callers, but that's it. > So we can report a conservative estimate of *num_same?
Yes. We can set *num_same to the number of bits from sector_num to the end of the bitmap sector. > 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. To be as efficient as possible, you could keep a memory copy of the bitmap, so that you only have to do writes, not reads. The memory copy would be somewhat expensive of course but perhaps reasonable (256M of memory for a 1T image). The largest cost would be loading the bitmap from memory at startup. But that can be done later. Paolo
