Re: [PATCH] Allow userspace block device implementation

[Zachary Amsden <zamsden@redhat.com>]

Alan Cox wrote:
>> Somehow this made me think of FUSE/CUSE... should this be named aBUSE?
>> Oh wait it is :-), what I'm after is I guess is, can we share some of
>> the FUSE/CUSE code?

Well, it is A Block device in User SpacE :)  I don't think there is a
lot of code sharing benefit in some 800 odd lines, but I could be wrong.

> It reminds me of the existing and perfectly functional network block
> device (nbd) we already have and which has also been present for years.

Yes, I agree, in fact I looked at nbd as I was writing this, but I
believe it is different enough to warrant further investigation.

The network block device requires access to a socket, which the code at
least seems to imply brings up the potential for deadlocks when
self-hosting.  This was designed to explicitly support self-hosting.

This device can be used without CONFIG_NET (not a big advantage, I
agree), and is completely connectionless, which I would argue is a big
advantage.

NBD is perfectly functional, but it seemed more complicated than
necessary for a purely local implementation.  A fully functional null
server (just returns zeros, full error checking and normal whitespace)
can be implemented in about 60 lines of C code, which I don't think is
the case for NBD.  Of course, I'm sure it is possible with PERL bindings
as a one-liner, but the fundamental argument isn't about lines, it's
about complexity.  NBD requires socket allocation, listening and
connection; this requires only opening of a device node.

Can you swap over NBD?  Assuming one had pinned the userspace program
and it pre-allocated all memory so no pagein / alloc was required, would
it be deadlock proof?  I believe there are structure allocations
required for the socket implementation that go beyond the basic BIO
allocations, therefore making it impossible.  In /theory/, one should be
able to swap over this device.  In practice, it's probably a really bad
idea.

It seems then that NBD is a strict subset of the functionality provided
by this type of module.

Zach