Re: [Qemu-devel] [PATCH v10 01/10] allow writing to the backing file

[Wen Congyang <wency@cn.fujitsu.com>]

On 10/10/2015 03:07 AM, Eric Blake wrote:
> 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 <wency@cn.fujitsu.com>
>> Signed-off-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
>> Signed-off-by: Gonglei <arei.gonglei@huawei.com>
>> ---
>>  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.

I try to reopen the backing files for write when starting block replication.
I tested it, and it doesn't work.
Here is my usage:
command line:
-drive if=none,id=colo-disk1,file.filename=/data/images/kvm/suse/temp.img,driver=raw
-drive if=virtio,id=active-disk1,driver=replication,mode=secondary,file.driver=qcow2,file.file.filename=/mnt/ramfs/active_disk.img,file.backing.driver=qcow2,file.backing.file.filename=/mnt/ramfs/hidden_disk.img,file.backing.backing.file.filename=/data/images/kvm/suse/suse11_3.img,file.backing.backing.driver=raw,file.backing.backing.node-name=sdisk

{'execute': 'blockdev-remove-medium', 'arguments': {'device': 'colo-disk1'} }
{'execute': 'blockdev-insert-medium', 'arguments': {'device': 'colo-disk1', 'node-name': 'sdisk'} }
{'execute': 'nbd-server-start', 'arguments': {'addr': {'type': 'inet', 'data': {'host': '192.168.3.1', 'port': '8889'} } } }
{'execute': 'nbd-server-add', 'arguments': {'device': 'colo-disk1', 'writable': true } }

All qmp command success, but the disk exported to nbd server is readonly even if I specify 
'writable': true in the QMP commad. The reason is that the BDS is readonly.

> 
>> +#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.
> 

Yes, I know it, but I don't know how to do it. Update BlockdevOptions?

Thanks
Wen Congyang