lkml.org 
[lkml]   [2009]   [Aug]   [28]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    From
    Date
    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..."
    ^^^^^^^^
    with
    (*) 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' :-)

    NeilBrown

    > 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 <pavel@ucw.cz>
    >
    > 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.
    >


    \
     
     \ /
      Last update: 2009-08-28 09:37    [W:0.045 / U:17.224 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site