On Thu, Apr 17, 2025 at 01:39:09PM -0500, Eric Blake wrote: > There are some optimizations that require knowing if an image starts > out as reading all zeroes, such as making blockdev-mirror faster by > skipping the copying of source zeroes to the destination. The > existing bdrv_co_is_zero_fast() is a good building block for answering > this question, but it tends to give an answer of 0 for a file we just > created via QMP 'blockdev-create' or similar (such as 'qemu-img create > -f raw'). Why? Because file-posix.c insists on allocating a tiny > header to any file rather than leaving it 100% sparse, due to some > filesystems that are unable to answer alignment probes on a hole. But > teaching file-posix.c to read the tiny header doesn't scale - the > problem of a small header is also visible when libvirt sets up an NBD > client to a just-created file on a migration destination host. > > So, we need a wrapper function that handles a bit more complexity in a > common manner for all block devices - when the BDS is mostly a hole, > but has a small non-hole header, it is still worth the time to read > that header and check if it reads as all zeroes before giving up and > returning a pessimistic answer. > > Signed-off-by: Eric Blake <ebl...@redhat.com> > --- > include/block/block-io.h | 2 ++ > block/io.c | 58 ++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 60 insertions(+) > > diff --git a/include/block/block-io.h b/include/block/block-io.h > index b49e0537dd4..b99cc98d265 100644 > --- a/include/block/block-io.h > +++ b/include/block/block-io.h > @@ -161,6 +161,8 @@ bdrv_is_allocated_above(BlockDriverState *bs, > BlockDriverState *base, > > int coroutine_fn GRAPH_RDLOCK > bdrv_co_is_zero_fast(BlockDriverState *bs, int64_t offset, int64_t bytes); > +int coroutine_fn GRAPH_RDLOCK > +bdrv_co_is_all_zeroes(BlockDriverState *bs); > > int GRAPH_RDLOCK > bdrv_apply_auto_read_only(BlockDriverState *bs, const char *errmsg, > diff --git a/block/io.c b/block/io.c > index 6ef78070915..dc1341e4029 100644 > --- a/block/io.c > +++ b/block/io.c > @@ -2778,6 +2778,64 @@ int coroutine_fn bdrv_co_is_zero_fast(BlockDriverState > *bs, int64_t offset, > return 1; > } > > +/* > + * Check @bs (and its backing chain) to see if the entire image is known > + * to read as zeroes. > + * Return 1 if that is the case, 0 otherwise and -errno on error. > + * This test is meant to be fast rather than accurate so returning 0 > + * does not guarantee non-zero data; however, it can report 1 in more
False negatives are possible, let's also document that false positives
are not possible:
This test is mean to be fast rather than accurate so returning 0 does
not guarantee non-zero data, but returning 1 does guarantee all zero
data; ...
> + * cases than bdrv_co_is_zero_fast.
> + */
> +int coroutine_fn bdrv_co_is_all_zeroes(BlockDriverState *bs)
> +{
> + int ret;
> + int64_t pnum, bytes;
> + char *buf;
> + QEMUIOVector local_qiov;
> + IO_CODE();
> +
> + bytes = bdrv_co_getlength(bs);
> + if (bytes < 0) {
> + return bytes;
> + }
> +
> + /* First probe - see if the entire image reads as zero */
> + ret = bdrv_co_common_block_status_above(bs, NULL, false, BDRV_BSTAT_ZERO,
> + 0, bytes, &pnum, NULL, NULL,
> + NULL);
> + if (ret < 0) {
> + return ret;
> + }
> + if (ret & BDRV_BLOCK_ZERO) {
> + return bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
> + }
> +
> + /*
> + * Because of the way 'blockdev-create' works, raw files tend to
> + * be created with a non-sparse region at the front to make
> + * alignment probing easier. If the block starts with only a
> + * small allocated region, it is still worth the effort to see if
> + * the rest of the image is still sparse, coupled with manually
> + * reading the first region to see if it reads zero after all.
> + */
> + if (pnum > qemu_real_host_page_size()) {
Probably not worth it for the corner case, but replacing
qemu_real_host_page_size() with 128 KiB would allow this to work on
images created on different CPU architectures (4 KiB vs 64 KiB page
sizes).
> + return 0;
> + }
> + ret = bdrv_co_is_zero_fast(bs, pnum, bytes - pnum);
> + if (ret <= 0) {
> + return ret;
> + }
> + /* Only the head of the image is unknown, and it's small. Read it. */
> + buf = qemu_blockalign(bs, pnum);
> + qemu_iovec_init_buf(&local_qiov, buf, pnum);
> + ret = bdrv_driver_preadv(bs, 0, pnum, &local_qiov, 0, 0);
> + if (ret >= 0) {
> + ret = buffer_is_zero(buf, pnum);
> + }
> + qemu_vfree(buf);
> + return ret;
> +}
> +
> int coroutine_fn bdrv_co_is_allocated(BlockDriverState *bs, int64_t offset,
> int64_t bytes, int64_t *pnum)
> {
> --
> 2.49.0
>
>
signature.asc
Description: PGP signature
