Pavel
---

From: Theodore Tso <tytso@mit.edu>

Document that many devices are too broken for filesystems to protect
data in case of powerfail.

Signed-of-by: Pavel Machek [off-list ref]

diff --git a/Documentation/filesystems/dangers.txt

b/Documentation/filesystems/dangers.txt
new file mode 100644
index 0000000..2f3eec1

--- /dev/null
+++ b/Documentation/filesystems/dangers.txt

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+There are storage devices that high highly undesirable properties when
+they are disconnected or suffer power failures while writes are in
+progress; such devices include flash devices and 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 these devices 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.
+
+(*) Degraded array or single disk failure "near" the powerfail is
+neccessary for this property of RAID arrays to bite.