[Nbd] copy on write breakage

[Phil Howard <phil-nbd-general@...98...>]

I just tried copy on write (-c option or copyonwrite config).  I think this
needs a way to specify at least where the file will be written, if not even
the exact file name itself.  I propose either overloading the existing option
or adding a new option to allow specifying a path.  If the path is a directory
then a generated-name file will be created within that directory.  If the path
is a non-existing name then create file and use that.  If the file actually
exists, that may be more complicated (e.g. supporting previously written copy
on write file).  But at least for now I'd like to see a way to specify where.

Currently, nbd-server tries to create the copy on write file in "/dev".  On my
system, and an increasing number of others, "/dev" tmpfs, ramfs, or rootfs.
This will also be seen soon on embedded systems (something I am now working on).
There is also the permission issue to access "/dev" to create files when the
effective uid nbd-server is running under is non-root.

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| Phil Howard KA9WGN       | http://linuxhomepage.com/      http://ham.org/ |
| (first name) at ipal.net | http://phil.ipal.org/   http://ka9wgn.ham.org/ |
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