Re: [patch] document flash/RAID dangers

From: Pavel Machek
Date: Tue Aug 25 2009 - 20:07:22 EST

On Tue 2009-08-25 19:48:09, Ric Wheeler wrote:
>> ---
>> 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 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.
> I would strike the entire mention of MD devices since it is your
> assertion, not a proven fact. You will cause more data loss from common

That actually is a fact. That's how MD RAID 5 is designed. And btw
those are originaly Ted's words.

> events (single sector errors, complete drive failure) by steering people
> away from more reliable storage configurations because of a really rare
> edge case (power failure during split write to two raid members while
> doing a RAID rebuild).

I'm not sure what's rare about power failures. Unlike single sector
errors, my machine actually has a button that produces exactly that
event. Running degraded raid5 arrays for extended periods may be
slightly unusual configuration, but I suspect people should just do
that for testing. (And from the discussion, people seem to think that
degraded raid5 is equivalent to raid0).

>> 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.
> This is very misleading. All storage "can" have silent data loss, you are
> making a statement without specifics about frequency.

substitute with "can (by design)"?

Now, if you can suggest useful version of that document meeting your

(cesky, pictures)
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