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Re: [Qemu-block] [Qemu-devel] [PATCH 03/16] allow writing to the backing
From: |
Eric Blake |
Subject: |
Re: [Qemu-block] [Qemu-devel] [PATCH 03/16] allow writing to the backing file |
Date: |
Wed, 2 Sep 2015 10:06:02 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.1.0 |
On 09/02/2015 02:51 AM, Wen Congyang wrote:
> Signed-off-by: Wen Congyang <address@hidden>
> Signed-off-by: zhanghailiang <address@hidden>
> Signed-off-by: Gonglei <address@hidden>
Not much description in the commit message. I really want an
explanation of why this patch is necessary. After all, with
'block-commit', we were able to turn on read-write mode of backing files
on an as-needed basis, without having to expose that to the end user.
Giving the end user a knob that they must tune feels a bit awkward, and
probably means we don't have the design right.
> ---
> block.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> qapi/block-core.json | 7 ++++++-
> 2 files changed, 46 insertions(+), 2 deletions(-)
>
> +#define ALLOW_WRITE_BACKING_FILE "allow-write-backing-file"
> +static QemuOptsList backing_file_opts = {
> + .name = "backing_file",
> + .head = QTAILQ_HEAD_INITIALIZER(backing_file_opts.head),
> + .desc = {
> + {
> + .name = ALLOW_WRITE_BACKING_FILE,
> + .type = QEMU_OPT_BOOL,
> + .help = "allow write to backing file",
If you do add more justification for why this patch is necessary, then,
s/write/writes/
> +++ b/qapi/block-core.json
> @@ -1408,6 +1408,10 @@
> # @detect-zeroes: #optional detect and optimize zero writes (Since 2.1)
> # (default: off)
> #
> +# @allow-write-backing-file: #optional whether the backing file is opened in
> +# read-write mode. It is only for backing file
> +# (Since 2.5 default: false)
> +#
The name feels a bit long.
It sounds like it is an error to pass allow-write-backing-file for a
top-level BDS (that is, the BDS associated with a BB). Meanwhile, the
default for any backing chain BDS is to open it read-only, regardless of
the 'read-only' setting of the parent. But can we just allow
'read-only':false on a backing BDS to mean that the BDS starts life as
read-write, without having to add a new parameter?
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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