Am 21.11.2011 11:53, schrieb Stefan Hajnoczi: > On Mon, Nov 21, 2011 at 10:44 AM, Kevin Wolf <kw...@redhat.com> wrote: >> Am 18.11.2011 15:34, schrieb Stefan Hajnoczi: >>> 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. >> >> qcow1 doesn't work on block devices anyway. > > Okay, both of my original points were moot, Kevin and Paolo have explained > why: > > The L1 RAM size issue doesn't really matter since we hold the entire > L1 in RAM during normal operation anyway. Holding it in RAM during > creation is no worse. > > The zero initialization could be optimized as Paolo suggested with > truncate since qcow1 always works on image files (which have automatic > zero initialization).
I didn't say this. :-) At least in theory, block devices may not be the only protocols with !has_zero_init. We have only covered raw-posix with this discussion. I would prefer an explicit write of the table to avoid breaking other protocols (though I don't think we have one today; curl would be a candidate, but it is read-only). Kevin