Re: [PATCH v3 04/11] block: Add new bdrv_co_is_all_zeroes() function

[Stefan Hajnoczi <stefanha@redhat.com>]

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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 <eblake@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 <stefanha@redhat.com>

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