linux-kernel - Re: [PATCH] Allow userspace block device implementation

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Message-ID: <4A80A37C.9000808@redhat.com>
Date:	Mon, 10 Aug 2009 12:47:24 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Pavel Machek <pavel@....cz>
CC:	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	axboe@...nel.dk, hch@...radead.org, akpm@...ux-foundation.org,
	Paul.Clements@...eleye.com, tytso@....edu
Subject: Re: [PATCH] Allow userspace block device implementation

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
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