Re: [Libguestfs] nbdkit test-checkwrite.sh failing occasionally on ppc64le
On Tue, Jul 19, 2022 at 04:45:53PM +0100, Richard W.M. Jones wrote:
qemu-img has new(?) behaviour that breaks some things:
$ nbdkit linuxdisk ~/d/nbdkit --filter=checkwrite --run 'qemu-img convert
"$uri" "$uri"'
nbdkit: linuxdisk.0: error: data written does not match expected
qemu-img: nbd://localhost:10809: error while converting raw: Failed to clear the new
image's first sector: Input/output error
I understand why this happens, but also it's kind of a bug ... We
should probably modify checkwrite to handle this, but it's rather
complicated.
I see you fixed this in the meantime (commit c7420c04), but repeating
the context here: qemu-img convert normally assumes that source and
destination are not the same, and assumes that when creating a
destination on an existing block device (such as NBD), it must wipe
any pre-existing signature in the first sector of the destination to
make sure we don't probe an incorrect format. It is this attempt to
wipe that writes different data; using 'qemu-img convert -n' skips the
creation step and therefore the wipe.
Perhaps qemu-img convert could be taught to special-case when -n is
not in use by first reading the destination to see if it reads
identically and only do the wipe if not; but that's asking a lot when
we can solve it by using -n.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org