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Commit f3ef6f63 authored by Karsten Wiese's avatar Karsten Wiese Committed by Linus Torvalds
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[PATCH] Speedup FAT filesystem directory reads



      OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>

This speeds up directory reads for large FAT partitions, if the buffercache
has to be filled from the drive. Following values were taken from:

        $ time find path_to_freshly_mounted_fat > /dev/null

on an otherwise idle system.

FAT with 16KB Clusters on IDE attached drive:   Factor  2
FAT with 32KB Clusters on USB2 attached drive:  Factor 10 (!)
Its less than 1/10 slower, if the buffercache is uptodate.

The patch introduces the new function fat_dir_readahead().

fat_dir_readahead() calls sb_breadahead() to readahead a whole cluster,
if the requested sector is the first one in a cluster.
It is usefull to do this, because on FAT directories occupy whole
clusters, with the exception of FAT12/FAT16 root dirs.

Readahead is only done, if the cluster's first sector is not uptodate
to avoid overhead, when the buffer cache is already uptodate.
Note that under memory pressure, the maximal byte count wasted
(read: has to be red from disk twice) is 1 cluster's size.  Thats 64KB.

fat_dir_readahead() is called from fat__get_entry().

There is also an unrelated cleanup at one spot:

        if (bh)
                brelse(bh);

is replaced with:

        brelse(bh);

brelse() can handle NULL pointer arguments by itself.

Signed-off-by: default avatarKarsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: default avatarOGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: default avatarAndrew Morton <akpm@osdl.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@osdl.org>
parent 378bac82
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