Re: Query on the dirty bitmap

[prashant patil <pmgpatil@gmail.com>]

[-- Attachment #1: Type: text/plain, Size: 2092 bytes --]

I was trying to read the bitmap of the running vm's disk. When I followed
below mentioned commands, then I was able to read the bitmap properly.
block-dirty-bitmap-add, block-dirty-bitmap-disable, nbd-server-start,
nbd-server-add, qemu-img
map with x-dirty-bitmap image-opts.

Please let me know if there is any other better alternative to do the same.

Thanks
Prashant

On Fri, Feb 28, 2025 at 3:53 AM Eric Blake <eblake@redhat.com> wrote:

> On Wed, Feb 19, 2025 at 04:23:26PM +0530, prashant patil wrote:
> > Hello All,
> > Hope this email finds you well.
> >
> > I have been trying with qemu for a while now, and have come across a
> > problem specific to dirty bitmaps. I have enabled bitmap on the qcow2
> disk
> > image using 'qemu-img bitmap' command, exposed the bitmap over a unix
> > socket using 'qemu-nbd' command. Now when I try to read the bitmap using
> > 'qemu-img map' command with 'x-dirty-bitmap=qemu:dirty-bitmap:{bitmap}'
> > option, I get one single extent which shows that the entire disk is
> dirty.
> > Note that the disk size is 5 GB, and has only a few MB of data in it, and
> > had added very small data after the bitmap was enabled. Bitmap output has
> > been pasted below.
>
> Can you show the exact sequence of command lines you used to create
> the image, dirty a portion of it, then start up the qemu-nbd process
> to inspect it?  As written, I can't reproduce your issue, but I know
> it sounds similar to tests/qemu-iotests/tests/qemu-img-bitmaps which
> does what you're talking about, so I know the code works and have to
> suspect you may have missed a step or reordered things in such a way
> that the entire bitmap is reading as dirty.
>
> >
> > [{ "start": 0, "length": 5368709120, "depth": 0, "present": true, "zero":
> > false, "data": true, "compressed": false, "offset": 0}]
> >
> > Can someone please help me understand why the bitmap content shows the
> > entire disk as dirty?
>
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.
> Virtualization:  qemu.org | libguestfs.org
>
>

[-- Attachment #2: Type: text/html, Size: 2829 bytes --]