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

[Michael Tokarev]

On 03.07.2012 15:05, address@hidden wrote:
> From: Wenchao Xia <address@hidden>
> 
>   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 <address@hidden>
> ---
>  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.
So, if you specify the format of the device explicitly, by using
format=raw, only single connection will be made.

/mjt