Re: [Qemu-devel] [PATCH] add text about how to use qemu-nbd with qemu

[Kevin Wolf <kwolf@redhat.com>]

Am 03.07.2012 21:45, schrieb Michael Tokarev:
> On 03.07.2012 15:05, xiawenc@linux.vnet.ibm.com wrote:
>> From: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
>>
>>   Qemu system emulator reports only fails that make people confused
>> about why, when it is invoked with nbd block device tring to connect
>> qemu-nbd server. In fact qemu will try connect server for several
>> times but server only accept one connect by default.
>>   I paid some times to find out why it fails. This patch add special
>> text to explain how to use it in such case.
>>
>> Signed-off-by: Wenchao Xia <xiawenc@linux.vnet.ibm.com>
>> ---
>>  qemu-nbd.texi |    6 ++++++
>>  1 files changed, 6 insertions(+), 0 deletions(-)
>>
>> diff --git a/qemu-nbd.texi b/qemu-nbd.texi
>> index 6955d90..014040a 100644
>> --- a/qemu-nbd.texi
>> +++ b/qemu-nbd.texi
>> @@ -44,6 +44,12 @@ Export QEMU disk image using NBD protocol.
>>    display this help and exit
>>  @item -V, --version
>>    output version information and exit
>> +
>> +  Note: When qemu-nbd was used to export a disk that would be used by QEMU
>> +block device, -t or --persistent must be set for that QEMU nbd client would try
>> +connect more than one time. For eg:
>> +  qemu-nbd ./simple.img -p 10809 -t
>> +  qemu -hdb nbd:127.0.0.1:10809
> 
> It is not accurate.
> 
> qemu connect to nbd server not "several" but exactly two times by default.
> The first connection it does (it need not be a separate connection
> actually, and this, I'd say, a bug in qemu) in order to guess the format
> of the internal structure inside - be it raw, or qcow, or anything else.

I think I agree that we should get rid of this behaviour. It's going to
bite us with fd passing as well. It should be possible to open the
protocol first and the use that open BlockDriverState both for probing
and for the real open.

Kevin