Squashfs: Add an option to set dev block size to 4K

This commit adds an option to set the device block size used to 4K.

By default Squashfs sets the device block size (sb_min_blocksize) to 1K
or the smallest block size supported by the block device (if larger).
This, because blocks are packed together and unaligned in Squashfs,
should reduce latency.

This, however, gives poor performance on MTD NAND devices where
the optimal I/O size is 4K (even though the devices can support
smaller block sizes).

Using a 4K device block size may also improve overall I/O
performance for some file access patterns (e.g. sequential
accesses of files in filesystem order) on all media.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>

3 files changed, 30 insertions, 1 deletions

diff --git a/fs/squashfs/Kconfig b/fs/squashfs/Kconfig
index 1360d4f88f41..5586011b6a38 100644
--- a/fs/squashfs/Kconfig
+++ b/fs/squashfs/Kconfig
@@ -78,6 +78,28 @@ config SQUASHFS_XZ
 
           If unsure, say N.
 
+config SQUASHFS_4K_DEVBLK_SIZE
+        bool "Use 4K device block size?"
+        depends on SQUASHFS
+        help
+          By default Squashfs sets the dev block size (sb_min_blocksize)
+          to 1K or the smallest block size supported by the block device
+          (if larger).  This, because blocks are packed together and
+          unaligned in Squashfs, should reduce latency.
+
+          This, however, gives poor performance on MTD NAND devices where
+          the optimal I/O size is 4K (even though the devices can support
+          smaller block sizes).
+
+          Using a 4K device block size may also improve overall I/O
+          performance for some file access patterns (e.g. sequential
+          accesses of files in filesystem order) on all media.
+
+          Setting this option will force Squashfs to use a 4K device block
+          size by default.
+
+          If unsure, say N.
+
 config SQUASHFS_EMBEDDED
         bool "Additional option for memory-constrained systems"
         depends on SQUASHFS
diff --git a/fs/squashfs/squashfs_fs.h b/fs/squashfs/squashfs_fs.h
index b4a4e539a08c..e8e14645de9a 100644
--- a/fs/squashfs/squashfs_fs.h
+++ b/fs/squashfs/squashfs_fs.h
@@ -36,6 +36,13 @@
 #define SQUASHFS_FILE_SIZE              131072
 #define SQUASHFS_FILE_LOG               17
 
+/* default size of block device I/O */
+#ifdef CONFIG_SQUASHFS_4K_DEVBLK_SIZE
+#define SQUASHFS_DEVBLK_SIZE 4096
+#else
+#define SQUASHFS_DEVBLK_SIZE 1024
+#endif
+
 #define SQUASHFS_FILE_MAX_SIZE          1048576
 #define SQUASHFS_FILE_MAX_LOG           20
 
diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index 7438850c62d0..2da1715452ac 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -95,7 +95,7 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
         }
         msblk = sb->s_fs_info;
 
-        msblk->devblksize = sb_min_blocksize(sb, BLOCK_SIZE);
+        msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE);
         msblk->devblksize_log2 = ffz(~msblk->devblksize);
 
         mutex_init(&msblk->read_data_mutex);