dm: add bufio

The dm-bufio interface allows you to do cached I/O on devices,
holding recently-read blocks in memory and performing delayed writes.

We don't use buffer cache or page cache already present in the kernel, because:
* we need to handle block sizes larger than a page
* we can't allocate memory to perform reads or we'd have deadlocks

Currently, when a cache is required, we limit its size to a fraction of
available memory.  Usage can be viewed and changed in
/sys/module/dm_bufio/parameters/ .

The first user is thin provisioning, but more dm users are planned.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index f75a66e7d312..b1a921497043 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -208,6 +208,14 @@ config DM_DEBUG
 
           If unsure, say N.
 
+config DM_BUFIO
+       tristate
+       depends on BLK_DEV_DM && EXPERIMENTAL
+       ---help---
+         This interface allows you to do buffered I/O on a device and acts
+         as a cache, holding recently-read blocks in memory and performing
+         delayed writes.
+
 config DM_CRYPT
         tristate "Crypt target support"
         depends on BLK_DEV_DM