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. Cheers, Andreas On 2011-06-17, at 8:20 AM, Eric Sandeen <sandeen@xxxxxxxxxx> wrote: > 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 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
