On Mon, Apr 22, 2013 at 5:18 PM, Liu Yuan <namei.u...@gmail.com> wrote: > On 04/22/2013 11:03 PM, Stefan Hajnoczi wrote: >> Imagine sector_num = 0 and nb_sectors = SD_DATA_OBJ_SIZE / BDRV_SECTOR_SIZE. >> >> start = 0 >> end = 1 >> >> You don't want object 1, only object 0. > > Hmm, math, ouch. So nb_sectors include sector_num? What I mean is > > If [start, end] mean a range of sectors,so > > 1. nb_sectors = end - start + 1 (I assume this one) > 2. nb_sectors = end - start (you meant this one?) > > which one is correct for .co_is_allocated?
The first sector is included in nb_sectors. Mathematically the range is defined as [sector_num, sector_num + nb_sectors). Notice the half-open interval, sector_num + nb_sectors is excluded. The last included sector is sector_num + nb_sectors - 1. You can look at qcow2's implementation, especially count_contiguous_clusters() to double-check. Stefan
