ext4: make trim/discard optional (and off by default)

It is anticipated that when sb_issue_discard starts doing
real work on trim-capable devices, we may see issues.  Make
this mount-time optional, and default it to off until we know
that things are working out OK.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 6 insertions, 0 deletions

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 6d94e0696f8c..26904ff6d61b 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -353,6 +353,12 @@ noauto_da_alloc		replacing existing files via patterns such as
                         system crashes before the delayed allocation
                         blocks are forced to disk.
 
+discard         Controls whether ext4 should issue discard/TRIM
+nodiscard(*)            commands to the underlying block device when
+                        blocks are freed.  This is useful for SSD devices
+                        and sparse/thinly-provisioned LUNs, but it is off
+                        by default until sufficient testing has been done.
+
 Data Mode
 =========
 There are 3 different data modes: