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>

Documentation/filesystems/ext4.txt

@@ -177,6 +177,11 @@ barrier=<0|1(*)>	This enables/disables the use of write barriers in
 			your disks are battery-backed in one way or another,
 			disabling barriers may safely improve performance.
 
+inode_readahead=n	This tuning parameter controls the maximum
+			number of inode table blocks that ext4's inode
+			table readahead algorithm will pre-read into
+			the buffer cache.  The default value is 32 blocks.
+
 orlov		(*)	This enables the new Orlov block allocator. It is
 			enabled by default.
 
@@ -252,6 +257,7 @@ stripe=n		Number of filesystem blocks that mballoc will try
 delalloc	(*)	Deferring block allocation until write-out time.
 nodelalloc		Disable delayed allocation. Blocks are allocation
 			when data is copied from user to page cache.
+
 Data Mode
 =========
 There are 3 different data modes:

Documentation/filesystems/proc.txt

@@ -956,6 +956,9 @@ Table 1-10: Files in /proc/fs/ext4/<devname>
                  files are packed closely together.  Each large file
                  will have its blocks allocated out of its own unique
                  preallocation pool.
+inode_readahead  Tuning parameter which controls the maximum number of
+                 inode table blocks that ext4's inode table readahead
+                 algorithm will pre-read into the buffer cache
 ..............................................................................