File 0125-mdadm-Add-description-of-write-journal-to-md.4.patch of Package mdadm.5365

From 28f83f6d3b1b3b386e60b45848b9284e2238f77f Mon Sep 17 00:00:00 2001
From: Song Liu <songliubraving@fb.com>
Date: Wed, 21 Oct 2015 11:35:15 -0700
Subject: [PATCH 179/359] mdadm: Add description of write journal to md.4
References: bsc#1081910

Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Coly Li <colyli@suse.de>

---
 md.4 | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

Index: mdadm-3.3.1/md.4
===================================================================
--- mdadm-3.3.1.orig/md.4
+++ mdadm-3.3.1/md.4
@@ -579,6 +579,26 @@ The log is particularly useful when reco
 cannot be read from the other devices, the bulk of the recovery can
 complete and those few bad blocks will be recorded in the bad block log.
 
+.SS RAID456 WRITE JOURNAL
+
+Due to non-atomicity nature of RAID write operations, interruption of
+write operations (system crash, etc.) to RAID456 array can lead to
+inconsistent parity and data loss (so called RAID-5 write hole).
+
+To plug the write hole, from Linux 4.4 (to be confirmed),
+.I md
+supports write ahead journal for RAID456. When the array is created,
+an additional journal device can be added to the array through
+.IR write-journal
+option. The RAID write journal works similar to file system journals.
+Before writing to the data disks, md persists data AND parity of the
+stripe to the journal device. After crashes, md searches the journal
+device for incomplete write operations, and replay them to the data
+disks.
+
+When the journal device fails, the RAID array is forced to run in
+read-only mode.
+
 .SS WRITE-BEHIND
 
 From Linux 2.6.14,