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Date:	Mon, 9 Nov 2009 11:50:54 +0100
From:	Pavel Machek <>
To:	NeilBrown <>
Cc:	Ric Wheeler <>, Rob Landley <>,
	Theodore Tso <>, Florian Weimer <>,
	Goswin von Brederlow <>,
	kernel list <>,
	Andrew Morton <>,,,,,
Subject: Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:
 document conditions when reliable operation is possible)


> >> If you have a specific bug in MD code, please propose a patch.
> >
> > Interesting. So, what's technically wrong with the patch below?
> >
> You mean apart from ".... that high highly undesirable ...." ??
>                                ^^^^^^^^^^^

Ok, I still believe kernel documentation should be ... well... in
kernel, not in LWN article, so I fixed the patch according to your

Signed-off-by: Pavel Machek <>

diff --git a/Documentation/filesystems/dangers.txt b/Documentation/filesystems/dangers.txt
new file mode 100644
index 0000000..14d0324
--- /dev/null
+++ b/Documentation/filesystems/dangers.txt
@@ -0,0 +1,21 @@
+There are storage devices that have highly undesirable properties when
+they are disconnected or suffer power failures while writes are in
+progress; such devices include flash devices and degraded DM/MD RAID
+4/5/6 (*) arrays.  These devices have the property of potentially
+corrupting blocks being written at the time of the power failure, and
+worse yet, amplifying the region where blocks are corrupted such that
+additional sectors are also damaged during the power failure.
+Users who use such storage devices are well advised take
+countermeasures, such as the use of Uninterruptible Power Supplies,
+and making sure the flash device is not hot-unplugged while the device
+is being used.  Regular backups when using any devices, and these
+devices in particular is also a Very Good Idea.
+Otherwise, file systems placed on these devices can suffer silent data
+and file system corruption.  An forced use of fsck may detect metadata
+corruption resulting in file system corruption, but will not suffice
+to detect data corruption.
+(*) If device failure causes the array to become degraded during or
+immediately after the power failure, the same problem can result.

(cesky, pictures)
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