linux-kernel - Re: [PATCH] LogFS take three

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Date:	Thu, 17 May 2007 23:36:13 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	"Jörn Engel" <joern@...ybastard.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mtd@...ts.infradead.org, akpm@...l.org,
	Albert Cahalan <acahalan@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Greg KH <greg@...ah.com>, Ingo Oeser <ioe-lkml@...eria.de>
Subject: Re: [PATCH] LogFS take three

On Thursday 17 May 2007, Pekka Enberg wrote:
> 
> Jörn Engel wrote:
> > Compressing random data will actually enlarge it.  If that happens I
> > simply store the verbatim uncompressed data instead and mark it as such.
> > 
> > There is also demand for a user-controlled bit in the inode to disable
> > compression completely.  All those .jpg, .mpg, .mp3, etc. just waste
> > time by trying and failing to compress them.
> 
> So any sane way to enable compression is on per-inode basis which makes 
> 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 <><
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