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From: Arnd Bergmann <arnd@arndb.de>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Subject: Re: [PATCH] LogFS take three
Date: Thu, 17 May 2007 23:36:13 +0200
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Cc: akpm@osdl.org, Evgeniy Polyakov <johnpol@2ka.mipt.ru>,
	Albert Cahalan <acahalan@gmail.com>, Greg KH <greg@kroah.com>,
	=?utf-8?q?J=C3=B6rn?= Engel <joern@lazybastard.org>,
	linux-kernel@vger.kernel.org, Ingo Oeser <ioe-lkml@rameria.de>,
	linux-mtd@lists.infradead.org, Jan Engelhardt <jengelh@linux01.gwdg.de>,
	linux-fsdevel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
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On Thursday 17 May 2007, Pekka Enberg wrote:
>=20
> J=C3=B6rn Engel wrote:
> > Compressing random data will actually enlarge it. =C2=A0If that happens=
 I
> > simply store the verbatim uncompressed data instead and mark it as such.
> >=20
> > There is also demand for a user-controlled bit in the inode to disable
> > compression completely. =C2=A0All those .jpg, .mpg, .mp3, etc. just was=
te
> > time by trying and failing to compress them.
>=20
> So any sane way to enable compression is on per-inode basis which makes=20
> me still wonder why you need per-object compression.

1. it doesn't require user interaction, the file system will do the right
thing most of the time.

2. enlarging data is a very bad thing because it makes the behaviour
of the fs unpredictable. With uncompressed objects, you have a guaranteed
upper bound on the size.

	Arnd <><