Re: FYI: RAID5 unusably unstable through 2.6.14

[Helge Hafting <helge.hafting@aitel.hist.no>]

Cynbe ru Taren wrote:

> Just in case the RAID5 maintainers aren't aware of it:
>
> The current Linux kernel RAID5 implementation is just
> too fragile to be used for most of the applications
> where it would be most useful.
>
> In principle, RAID5 should allow construction of a
> disk-based store which is considerably MORE reliable
> than any individual drive.
>
> In my experience, at least, using Linux RAID5 results
> in a disk storage system which is considerably LESS
> reliable than the underlying drives.
>
> What happens repeatedly, at least in my experience over
> a variety of boxes running a variety of 2.4 and 2.6
> Linux kernel releases, is that any transient I/O problem
> results in a critical mass of RAID5 drives being marked
> 'failed', 
What kind of "transient io error" would that be?
That is not supposed to happen regularly. . .

You do replace failed drives immediately?  Allowing
systems to run "for a while" in degraded mode is
surely a recipe for disaster.  Degraded mode
has no safety at all, it is just raid-0 with a performance
overhead added in. :-/

Having hot spares is a nice way of replacing the failed
drive quickly.

> at which point there is no longer any supported
> way of retrieving the data on the RAID5 device, even
> though the underlying drives are all fine, and the underlying
> data on those drives almost certainly intact.
>  
As other have showed - "mdadm" can reassemble your
broken raid - and it'll work well in those cases where
the underlying drives indeed are ok.  It will fail
spectacularly if you have a real double fault though,
but then nothing short of raid-6 can save you.


Helge Hafting

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