On Thu, May 04, 2017 at 09:14:58PM -0500, Eric Blake wrote: > As mentioned in my 'qcow2 zero-cluster tweaks' cover letter > https://lists.gnu.org/archive/html/qemu-devel/2017-05/msg00722.html > I hit a case where we were not fully optimizing a write-zeroes > operation when we have a backing file that is shorter than the > active layer, and where the request crosses that boundary. I don't > know that the situation will be all that common (most qcow2 files > happen to be cluster-aligned in size; and most backing chains use > the same length at each layer of the chain), but for RFC purposes, > I wanted to explore how easy it would be to optimize the case > anyways, and to see if we can think of any other cases where > knowing that a block status crossed an end-of-file boundary can > be exploited for less work. > > Thus, this is a followup to that series, but I'm also okay if we > think it is too much maintenance compared to the potential gains, > and decide to drop it after all. > > Another potential use that I thought of: we may someday want to > track BDS length by bytes. Right now, bdrv_getlength() returns a > value in bytes, but which is sector-aligned, by adding padding; > but raw-format.c is able to see the transition between data up to > EOF followed by a hole after EOF. Because of that, my series to > convert bdrv_get_block_status() into a byte-based interface has to > fudge things - any time we see a mid-sector hole being reported, > we know it is because we hit EOF, and can round the result up to > the next sector boundary. Having the BDRV_BLOCK_EOF flag set in > that case may alert clients that the rounding took place, or help > us when we quit doing the rounding and actually start tracking BDS > lengths by bytes even where it is not sector-aligned. > > Eric Blake (2): > block: Add BDRV_BLOCK_EOF to bdrv_get_block_status() > block: Exploit BDRV_BLOCK_EOF for larger zero blocks > > include/block/block.h | 2 ++ > block/io.c | 42 +++++++++++++++++++++++++++++++++--------- > tests/qemu-iotests/154 | 4 ---- > tests/qemu-iotests/154.out | 12 ++++++------ > 4 files changed, 41 insertions(+), 19 deletions(-) > > -- > 2.9.3 > >
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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