linux-kernel - Re: Is there a "blackhole" /dev/null directory?

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Message-ID: <47B5E743.3000101@tmr.com>
Date:	Fri, 15 Feb 2008 14:25:55 -0500
From:	Bill Davidsen <davidsen@....com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
CC:	Andi Kleen <andi@...stfloor.org>,
	Jasper Bryant-Greene <jasper@...x.geek.nz>,
	rzryyvzy <rzryyvzy@...shmail.net>, linux-kernel@...r.kernel.org
Subject: Re: Is there a "blackhole" /dev/null directory?

Jan Engelhardt wrote:
> On Feb 14 2008 10:46, Andi Kleen wrote:
>> Jasper Bryant-Greene <jasper@...x.geek.nz> writes:
>>> This could be done fairly trivially with FUSE, and IMHO is a good use
>>> for FUSE because since you're just throwing most data away, performance
>>> is not a concern.
> 
> There is a much more interesting 'problem' with a "/dev/null directory".
> 
> Q: Why would you need such a directory?
> A: To temporarily fool a program into believing it wrote something.

Also: to let a program believe it was creating files which are used to 
hold the written data. Otherwise /dev/null would probably be the solution.
> 
> Q: Should all files disappear? (e.g. "unlink after open")
> A: Maybe not, programs may stat() the file right afterwards and
>    get confused by the "inexistence".

I think what is going to happen is that files created behave as if they 
are the result of a mknod resulting in a /dev/null clone.
> 
> Q: What if a program attempts to mkdir /dev/nullmnt/foo to just
>    create a file /dev/nullmnt/foo/barfile?
> A: /dev/nullmnt/foo must continue to exist or be accepted for a while,
>    or perhaps for eternity.

The directory structure can persist, it's the writing of data which can 
be avoided.

Real example:

A program which reads log files and prepares a whole raft of reports in 
a directory specified. If you just want to see the summary (stdout) and 
exception notices (stderr) having a nulldir would avoid the disk space 
and i/o load if you were just looking at the critical output rather than 
the analysis.

Yes, if this was an original program requirement it would or should have 
been a feature. Real world cases sometimes use tools in creative ways.

-- 
Bill Davidsen <davidsen@....com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
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