linux-ext4 - Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment

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Message-ID: <4E008045.1040909@sx.jp.nec.com>
Date:	Tue, 21 Jun 2011 20:28:05 +0900
From:	Kazuya Mio <k-mio@...jp.nec.com>
To:	Andreas Dilger <aedilger@...il.com>
CC:	Eric Sandeen <sandeen@...hat.com>, "Ted Ts'o" <tytso@....edu>,
	ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment_score()

2011/06/18 16:19, Andreas Dilger wrote:
> I was thinking about this, and am wondering if it makes sense to have an absolute score for fragmentation
> instead of a relative one?
>
> By absolute I mean something like fragments per MB or similar. A bad score might be anything>  1. For
> files smaller than 1 MB in size it would scale the ratio to the equivalent if the file was 1MB in size
> (e.g. a 16kB file with 4 fragments would have a score of 256, which is clearly bad).  Large files can
> have a score much less than 1, which is good.

I think fragments per MB is easy to understand. I will fix the library function
to "double e2p_get_fragscore(int fd)". To return fragments per MB, it will
get the number of extents and the total length of extents except the following
special cases:
- The extent whose initialize status is different from the next extent
- There is a hole between the extent and the next extent
- The extent is a tail

The output of filefrag would be as follows:

# filefrag /mnt/mp1/testfile
/mnt/mp1/testfile: 4 extents found, 0.75 fragments/MB

Regards,
Kazuya Mio

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