On Thu, Apr 24, 2025 at 07:52:04PM -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> > > --- > > v3: Use constant 128k as maximum data header size to read [Stefan] > --- > include/block/block-io.h | 2 ++ > block/io.c | 62 ++++++++++++++++++++++++++++++++++++++++ > 2 files changed, 64 insertions(+)
Reviewed-by: Stefan Hajnoczi <stefa...@redhat.com>
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