Re: raid is dangerous but that's secret (was Re: [patch] ext2/3:document conditions when reliable operation is possible)

From: Pavel Machek
Date: Fri Aug 28 2009 - 02:45:16 EST

On Thu 2009-08-27 21:32:49, Ric Wheeler wrote:
> On 08/27/2009 06:13 PM, Pavel Machek wrote:
>>>>> Repeat experiment until you get up to something like google scale or the
>>>>> other papers on failures in national labs in the US and then we can have an
>>>>> informed discussion.
>>>> On google scale anvil lightning can fry your machine out of a clear sky.
>>>> However, there are still a few non-enterprise users out there, and knowing
>>>> that specific usage patterns don't behave like they expect might be useful to
>>>> them.
>>> You are missing the broader point of both papers. They (and people like
>>> me when back at EMC) look at large numbers of machines and try to fix
>>> what actually breaks when run in the real world and causes data loss.
>>> The motherboards, S-ATA controllers, disk types are the same class of
>>> parts that I have in my desktop box today.
>> ...
>>> These errors happen extremely commonly and are what RAID deals with well.
>>> What does not happen commonly is that during the RAID rebuild (kicked
>>> off only after a drive is kicked out), you push the power button or have
>>> a second failure (power outage).
>>> We will have more users loose data if they decide to use ext2 instead of
>>> ext3 and use only single disk storage.
>> So your argument basically is
>> 'our abs brakes are broken, but lets not tell anyone; our car is still
>> safer than a horse'.
>> and
>> 'while we know our abs brakes are broken, they are not major factor in
>> accidents, so lets not tell anyone'.
>> Sorry, but I'd expect slightly higher moral standards. If we can
>> document it in a way that's non-scary, and does not push people to
>> single disks (horses), please go ahead; but you have to mention that
>> md raid breaks journalling assumptions (our abs brakes really are
>> broken).
> You continue to ignore the technical facts that everyone (both MD and
> ext3) people put in front of you.
> If you have a specific bug in MD code, please propose a patch.

Interesting. So, what's technically wrong with the patch below?


From: Theodore Tso <tytso@xxxxxxx>

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

Signed-of-by: Pavel Machek <pavel@xxxxxx>

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.

(cesky, pictures)
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