ext4: Use readahead when reading an inode from the inode table

With modern hard drives, reading 64k takes roughly the same time as
reading a 4k block.  So request readahead for adjacent inode table
blocks to reduce the time it takes when iterating over directories
(especially when doing this in htree sort order) in a cold cache case.
With this patch, the time it takes to run "git status" on a kernel
tree after flushing the caches via "echo 3 > /proc/sys/vm/drop_caches"
is reduced by 21%.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 2 insertions, 0 deletions

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 163c44527dde..922d18720c9e 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -790,6 +790,8 @@ static inline int ext4_valid_inum(struct super_block *sb, unsigned long ino)
 #define EXT4_DEF_RESUID         0
 #define EXT4_DEF_RESGID         0
 
+#define EXT4_DEF_INODE_READAHEAD_BLKS   32
+
 /*
  * Default mount options
  */