Date: Sat, 13 Mar 2004 20:48:27 +0100
From: =?iso-8859-1?Q?J=F6rn?= Engel <joern@wohnheim.fh-wedel.de>
To: Pavel Machek <pavel@ucw.cz>
Cc: Sytse Wielinga <s.b.wielinga@student.utwente.nl>,
       linux-kernel@vger.kernel.org
Subject: Re: [PATCH for testing] cow behaviour for hard links
Message-ID: <20040313194827.GA4748@wohnheim.fh-wedel.de>
References: <20040310193429.GB4589@wohnheim.fh-wedel.de> <200403121849.03505.s.b.wielinga@student.utwente.nl> <20040312182912.GB7087@wohnheim.fh-wedel.de> <20040313134330.GC3352@openzaurus.ucw.cz>
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On Sat, 13 March 2004 14:43:30 +0100, Pavel Machek wrote:
> 
> I do not know your current design, but...
> 
> In ideal world there would be no COW links. System would
> magically detect that you are doing cp -a, and would link
> at individual block level.
>
> Well, that would be probably too fs-specific. But introducing copyfile()
> syscall, which would just link the inodes if underlying fs
> supported it might be good start. On first
> write into one
> of linked files copy
> would be done...

Agreed.

> Only disadvantage I see is that such links would not survive
> tar-backup...

That's not a problem either.  Have a userspace program that checks all
files and hints for identical ones (new syscall, copyfile() cannot do
this without races).  Depending on fs size, the necessary data can
grow into the gigabytes, but the code is just 200 lines.

Or did you mean the problem of tar backups growing *much* larger than
the real filesystem?  Yes, tar becomes useless for backups then. :)

J?rn

-- 
Fancy algorithms are buggier than simple ones, and they're much harder
to implement. Use simple algorithms as well as simple data structures.
-- Rob Pike
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