Disable look-ahead for loop file.
Loopback files normally contain filesystems, in which case there are already
proper look-aheads in the upper layer, more look-aheads on the loopback file
only ruins the read-ahead hit rate.
Signed-off-by: Wu Fengguang <[email protected]>
---
I'd like to thank Tero Grundstr?m for uncovering the loopback problem.
drivers/block/loop.c | 6 ++++++
1 files changed, 6 insertions(+)
--- linux-2.6.16-rc6-mm2.orig/drivers/block/loop.c
+++ linux-2.6.16-rc6-mm2/drivers/block/loop.c
@@ -779,6 +779,12 @@ static int loop_set_fd(struct loop_devic
mapping = file->f_mapping;
inode = mapping->host;
+ /*
+ * The upper layer should already do proper look-ahead,
+ * one more look-ahead here only ruins the cache hit rate.
+ */
+ file->f_ra.flags |= RA_FLAG_NO_LOOKAHEAD;
+
if (!(file->f_mode & FMODE_WRITE))
lo_flags |= LO_FLAGS_READ_ONLY;
--