On 1/24/20 4:34 AM, David Edmondson wrote:
In many cases the target of a convert operation is a newly provisioned
target that the user knows is blank (filled with zeroes). In this
situation there is no requirement for qemu-img to wastefully zero out
the entire device.
Add a new option, --target-is-zero, allowing the user to indicate that
an existing target device is already zero filled.
Signed-off-by: David Edmondson <david.edmond...@oracle.com>
---
qemu-img-cmds.hx | 4 ++--
qemu-img.c | 25 ++++++++++++++++++++++---
qemu-img.texi | 4 ++++
3 files changed, 28 insertions(+), 5 deletions(-)
I'm working up a patch series that tries to auto-set this flag without
user interaction where possible (for example, if lseek(fd, 0, SEEK_DATA)
returns EOF, or if fstat() reports 0 blocks allocated, or if qcow2 sees
no L2 tables allocated, or a proposed extension to NBD passes on the
same...). I may rebase my series on top of your patch and tweak things
in yours accordingly.
But as it stands, the idea makes sense to me; even if we add ways for
some images to efficiently report initial state (and our existing
bdrv_has_zero_init() is NOT such a method), there are enough other
scenarios where the knob will be the only way to let qemu-img know the
intent.
+ case OPTION_TARGET_IS_ZERO:
+ /*
+ * The user asserting that the target is blank has the
+ * same effect as the target driver supporting zero
+ * initialisation.
Hmm. A git grep shows that 'initialization' has 200 hits,
'initialisation' has only 29. But I think it's a US vs. UK thing, so I
don't care which spelling you use.
@@ -2247,6 +2256,11 @@ static int img_convert(int argc, char **argv)
warn_report("This will become an error in future QEMU versions.");
}
+ if (s.has_zero_init && !skip_create) {
+ error_report("--target-is-zero requires use of -n flag");
+ goto fail_getopt;
+ }
+
Makes sense, although we could perhaps relax it to also work even when
the -n flag is supplied IF the destination image supports my proposal
for a new status bit set when an image is known to be opened with all
zero content.
s.src_num = argc - optind - 1;
out_filename = s.src_num >= 1 ? argv[argc - 1] : NULL;
@@ -2380,6 +2394,11 @@ static int img_convert(int argc, char **argv)
}
s.target_has_backing = (bool) out_baseimg;
+ if (s.has_zero_init && s.target_has_backing) {
+ error_report("Cannot use --target-is-zero with a backing file");
+ goto out;
+ }
+
Makes sense, although we could perhaps relax it to also work even when
there is a backing file IF the backing file supports my proposal for a
new status bit set when an image is known to be opened with all zero
content.
As my patch proposal is still not submitted, I'm fine if yours lands as-is:
Reviewed-by: Eric Blake <ebl...@redhat.com>
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org
