[PATCH] Update Documentation/md.txt to mention journaling won't help dirty+degraded case.

[Rob Landley]

From: Rob Landley <rob@xxxxxxxxxxx>

Add more warnings to the "Boot time assembly of degraded/dirty arrays" section,
explaining that using a journaling filesystem can't overcome this problem.

Signed-off-by: Rob Landley <rob@xxxxxxxxxxx>
---

 Documentation/md.txt |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/Documentation/md.txt b/Documentation/md.txt
index 4edd39e..52b8450 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -75,6 +75,23 @@ So, to boot with a root filesystem of a dirty degraded raid[56], use
 
    md-mod.start_dirty_degraded=1
 
+Note that Journaling filesystems do not effectively protect data in this
+case, because the update granularity of the RAID is larger than the journal
+was designed to expect.  Reconstructing data via partity information involes
+matching together corresponding stripes, and updating only some of these
+stripes renders the corresponding data in all the unmatched stripes
+meaningless.  Thus seemingly unrelated data in other parts of the filesystem
+(stored in the unmatched stripes) can become unreadable after a partial
+update, but the journal is only aware of the parts it modified, not the
+"collateral damage" elsewhere in the filesystem which was affected by those
+changes.
+
+Thus successful journal replay proves nothing in this context, and even a
+full fsck only shows whether or not the filesystem's metadata was affected.
+(A proper solution to this problem would involve adding journaling to the RAID
+itself, at least during degraded writes.  In the meantime, try not to allow
+a system to shut down uncleanly with its RAID both dirty and degraded, it
+can handle one but not both.)
 
 Superblock formats
 ------------------


-- 
Latency is more important than throughput. It's that simple. - Linus Torvalds
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