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

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Date:	Fri, 17 Jun 2011 09:20:55 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	"Ted Ts'o" <tytso@....edu>
CC:	Kazuya Mio <k-mio@...jp.nec.com>, ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 01/11 RESEND] libe2p: Add new function get_fragment_score()

On 6/16/11 10:18 PM, Ted Ts'o wrote:
> On Wed, Jun 15, 2011 at 03:33:19PM +0900, Kazuya Mio wrote:
>> This patch adds get_fragment_score() to libe2p. get_fragment_score() returns
>> the fragmentation score. It shows the percentage of extents whose size is
>> smaller than the input argument "threshold".
> 
> It perhaps might be useful to also articulate what are the goals of
> this metric.  Is just just to decide which files should be
> defragmented, and which should be left alone?  Or do you want to be
> able to compare which file is "worse off"?
> 
> I can imagine two files that have a score of 100%, but one is much
> worse off than the other.  Does that matter?  It may or might not,
> depending how you plan to use the fragmentation score, both now and in
> the future.  So it might be good to explicitly declare what are the
> goals for this metrics, and its planned use cases.
> 
> Regards,

Just as a random datapoint, the xfs_db "frag factor" has been a constant
source of misunderstanding and woe for us.  (Granted, it works differently;
it is an fs-wide number representing

	((actual - ideal) / actual)

extents in the fs.)

This "% of fragments smaller than threshold" is more easily understandable
and possibly more descriptive, but I think Ted makes good points;
think about how this will be used, and whether the metric is useful.

It's hard to make a single number a) make sense to the user, and b)
be usefully representative of fragmentation "badness" - so I am
feeling very cautious about this idea overall.

To really convey fragmentation "badness" you'd almost want a histogram
of fragment sizes, which is a bit hard to present concisely...

-Eric

> 						- Ted

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