Re: [Nbd] [RFC]Some questions about NBD

[Wouter Verhelst <w@...112...>]

[Paul and I both read the list; no need to add Cc]

On Sat, Feb 11, 2012 at 12:50:19AM +0800, harryxiyou wrote:
> Hi Wouter,
> 
> I have some questions to ask you, which are like following.
> 
> 1, Pavel said "Unlike NFS, it is possible to put any file system on it. But
>    (also unlike NFS), if someone has mounted NBD read/write, you must
>    assure that no one else will have it mounted. " on his own page. The
>    URL is http://atrey.karlin.mff.cuni.cz/~pavel/nbd/nbd.html . But i can not
>    understand '' if someone has mounted NBD read/write, you must
>    assure that no one else will have it mounted.". What is exact meaning of
>    this sentence.
>            One of my thoughts looks like this. We have two users, a1
> and a2 , who
>    have logined into a Linux OS. If a1 mounted NBD read/write, a2 must not
>    mount it currently, right?

Yeah, pretty much.

> 2, I have seen the main function of nbd-server.c, which version is nbd-2.9.15.
>    Some stuffs confuse me like following.
>        1> 1882 line: memset(pidftemplate, '\0', 256);
>             I just see pidftemplate is used in the daemon function.
> But i am not
>        sure what the pid file is used for.

So we can easily kill the server afterwards, if needs be.

> And also i don't know the meaning of this daemon function in nbd-server.c.

detach from stdin and stdout, and start serving as a background daemon.

>        2> 1873: SERVER *server;
>             If we have (server != NULL && (0 == server->port)) we will
> execute inetd
>        mode. I have searched the characters of inetd but i am not sure what the
>        meanings of inetd in our nbd-server.c do. And in what
> situations, we will do
>        the inetd mode, which should do what jobs?

inetd is an "internet superdaemon", used to run services without having
to have a separate process running for each and every service (at least,
as long as no client is using the service).

>        3> 1928: dousers();
>             I just tracked this function. It looks like it will set the
>             running group and user informations.

Correct.

> Why should we do this?

Because some people think it's a bad idea to run a service as root.

> Is it related to what
> pavel said
>        "if someone has mounted NBD read/write, you must assure that no one else
>         will have it mounted". Right ?

No.

-- 
The volume of a pizza of thickness a and radius z can be described by
the following formula:

pi zz a