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    SubjectRe: raid is dangerous but that's secret (was Re: [patch] ext2/3: document conditions when reliable operation is possible)
    On Fri, August 28, 2009 4:44 pm, Pavel Machek wrote:
    > On Thu 2009-08-27 21:32:49, Ric Wheeler wrote:
    >> 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 ...." ??

    And the phrase "Regular backups when using these devices ...." should
    be "Regular backups when using any devices .....".
    If you have a device failure near a power fail on a raid5 you might
    lose some blocks of data. If you have a device failure near (or not
    near) a power failure on raid0 or jbod etc you will certainly lose lots
    of blocks of data.

    I think it would be better to say:

    ".... and degraded DM/MD RAID 4/5/6(*) arrays..."
    (*) If device failure causes the array to become degraded during or
    immediately after the power failure, the same problem can result.

    And "necessary" only have the one 'c' :-)


    > Pavel
    > ---
    > From: Theodore Tso <>
    > Document that many devices are too broken for filesystems to protect
    > data in case of powerfail.
    > Signed-of-by: Pavel Machek <>
    > 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
    > @@ -0,0 +1,21 @@
    > +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.

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