On Fri, Nov 18, 2011 at 11:10 AM, Paolo Bonzini <pbonz...@redhat.com> wrote: > On 11/18/2011 11:59 AM, Stefan Hajnoczi wrote: >> >> + tmp = g_malloc0(sizeof(uint64_t)*l1_size); >> > + ret = bdrv_pwrite(qcow_bs, header_size, tmp, >> > sizeof(uint64_t)*l1_size); >> > + g_free(tmp); >> > + if (ret != sizeof(uint64_t)*l1_size) { >> > + goto exit; >> > } >> >> That means 400 MB of RAM for the zero L1 table for a 100 TB image. >> Since qcow is a legacy format this probably doesn't matter in practice >> but in theory this approach can require a noticable amount of RAM. > > 4 MB / TB is not a big deal (you probably would like the L1 table to be in > memory all the time), but why write the L1 table at all? Since the file was > CREATed, it is already zero and you can just leave a hole in the file.
I thought the same thing then remember sometimes people want to use image formats on block devices. I think at least making image creation not depend on has_zero_init is a good idea. Stefan