Re: [PATCH] Allow userspace block device implementation

[Zachary Amsden <zamsden@redhat.com>]

Pavel Machek wrote:
> Hi!

Hey!

> And yes, I believe that's show stopper. OTOH if you _can_ solve
> that... then you have some rather significant advantage over nbd.
> 
> (But guaranteeing progress for dirty writeout will be tricky even with
> mlocked userland, AFAICT...)

Actually, impossible, even with mlocked userland (*) which is what led
me to abandon going any further with it.  The problem is, to commit any
data, one must make a system call, thus consuming more resources.  It's
merely a toy, nothing more.  Sometimes it might be a useful toy, as nbd,
but nbd, being in kernel, has at least a better chance of solving the
swap problem.

(*) strictly speaking, it is possible to guarantee progress of the
device for read/write only to a finite region of mlocked memory and an
infinite region (limited only by size of off_t) of read-only data
computable with finite mlocked space.  Obviously, neither of these
(swap-to-ram), and (swap-over-ro-media) are actually useful for swap.

Zach