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Re: [Qemu-block] [PATCH v10 01/10] allow writing to the backing file
From: |
Eric Blake |
Subject: |
Re: [Qemu-block] [PATCH v10 01/10] allow writing to the backing file |
Date: |
Fri, 9 Oct 2015 13:07:30 -0600 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 09/25/2015 12:17 AM, Wen Congyang wrote:
> For block replication, we have such backing chain:
> secondary disk <-- hidden disk <-- active disk
> secondary disk is top BDS(use bacing reference), so it can be opened in
s/BDS(use bacing/BDS (use backing/
> read-write mode. But hidden disk is read only, and we need to write to
> hidden disk(backup job will write data to it).
s/disk(/disk (/
>
> TODO: support opening backing file in read-write mode if the BDS is
> created by QMP command blockdev-add.
>
> Signed-off-by: Wen Congyang <address@hidden>
> Signed-off-by: zhanghailiang <address@hidden>
> Signed-off-by: Gonglei <address@hidden>
> ---
> block.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 40 insertions(+), 1 deletion(-)
>
I really don't like this patch. We are able to automatically (re-)open
backing files for write during block-commit, without having to expose a
knob to the user then, so exposing a knob to the user here feels wrong.
> +#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 writes to backing file",
> + },
And even if we DO need this knob (which I doubt), you need corresponding
documentation of the knob in qapi/block-core.json, since we are trying
to keep the command line and QMP in sync when it comes to adding new
options.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
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