[PATCH] Use direct I/O for loop devices

[Demi Marie Obenour <demi@invisiblethingslab.com>]

This is a huge performance improvement for two reasons:

1. It uses the filesystem’s asynchronous I/O support, rather than using
   synchronous I/O.
2. It bypasses the page cache, removing a redundant layer of caching and
   associated overhead.
---
 tools/hotplug/Linux/block | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/hotplug/Linux/block b/tools/hotplug/Linux/block
index 2691b56951c9b82094471a141b9e0bed04abb929..75785f3a5422c4a5f962a4cd4f6acae5080d036d 100644
--- a/tools/hotplug/Linux/block
+++ b/tools/hotplug/Linux/block
@@ -330,7 +330,7 @@ mount it read-write in a guest domain."
         else
           roflag=''
         fi
-        do_or_die losetup $roflag "$loopdev" "$file"
+        do_or_die losetup --direct-io=on $roflag "$loopdev" "$file"
         xenstore_write "$XENBUS_PATH/node" "$loopdev"
         write_dev "$loopdev"
         release_lock "block"
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab