linux-kernel - Re: [PATCH 04/10] AXFS: axfs

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Message-ID: <48AEF2A3.7020905@lougher.demon.co.uk>
Date:	Fri, 22 Aug 2008 18:08:51 +0100
From:	Phillip Lougher <phillip@...gher.demon.co.uk>
To:	Arnd Bergmann <arnd@...db.de>
CC:	jaredeh@...il.com, Linux-kernel@...r.kernel.org,
	linux-embedded@...r.kernel.org,
	linux-mtd <linux-mtd@...ts.infradead.org>,
	Jörn Engel <joern@...fs.org>,
	tim.bird@...sony.com, cotte@...ibm.com, nickpiggin@...oo.com.au
Subject: Re: [PATCH 04/10] AXFS: axfs_inode.c

Arnd Bergmann wrote:
> On Friday 22 August 2008, Phillip Lougher wrote:
>>> This looks very nice, but could use some comments about how the data is
>>> actually stored on disk. It took me some time to figure out that it actually
>>> allows to do tail merging into compressed blocks, which I was about to suggest
>>> you implement ;-). Cramfs doesn't have them, and I found that they are the
>>> main reason why squashfs compresses better than cramfs, besides the default
>>> block size, which you can change on either one.
>> Squashfs has much larger block sizes than cramfs (last time I looked it 
>> was limited to 4K blocks), and it compresses the metadata which helps to 
>> get better compression.  But tail merging (fragments in Squashfs 
>> terminology) is obviously a major reason why Squashfs gets good compression.
> 
> The *default* block size in cramfs is smaller than in squashfs, but they both
> have user selectable block sizes. I found the impact of compressed metadata
> to be almost zero. 

Squashfs stores significantly more metadata than cramfs.  Remember 
cramfs has no support for filesystems > ~ 16Mbytes, no inode timestamps, 
truncates uid/gids, no hard-links, no nlink counts, no hashed 
directories,  no unique inode numbers.  If Squashfs didn't compress the 
metadata it would be significantly larger than cramfs.

Cheers

Phillip
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