Re: [patch] document flash/RAID dangers

From: Ric Wheeler
Date: Wed Aug 26 2009 - 07:58:51 EST

On 08/26/2009 07:21 AM, Pavel Machek wrote:
On Tue 2009-08-25 20:45:26, Ric Wheeler wrote:
On 08/25/2009 08:38 PM, Pavel Machek wrote:
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).
Power failures after a full drive failure with a split write during a rebuild?
Look, I don't need full drive failure for this to happen. I can just
remove one disk from array. I don't need power failure, I can just
press the power button. I don't even need to rebuild anything, I can
just write to degraded array.

Given that all events are under my control, statistics make little
sense here.
You are deliberately causing a double failure - pressing the power button
after pulling a drive is exactly that scenario.
Exactly. And now I'm trying to get that documented, so that people
don't do it and still expect their fs to be consistent.
The problem I have is that the way you word it steers people away from
RAID5 and better data integrity. Your intentions are good, but your text
is going to do considerable harm.

Most people don't intentionally drop power (or have a power failure)
during RAID rebuilds....
Example I seen went like this:

Drive in raid 5 failed; hot spare was available (no idea about
UPS). System apparently locked up trying to talk to the failed drive,
or maybe admin just was not patient enough, so he just powercycled the
array. He lost the array.

So while most people will not agressively powercycle the RAID array,
drive failure still provokes little tested error paths, and getting
unclean shutdown is quite easy in such case.

Then what we need to document is do not power cycle an array during a rebuild, right?

If it wasn't the admin that timed out and the box really was hung (no drive activity lights, etc), you will need to power cycle/reboot but then you should not have this active rebuild issuing writes either...

In the end, there are cascading failures that will defeat any data protection scheme, but that does not mean that the value of that scheme is zero. We need to be get more people to use RAID (including MD5) and try to enhance it as we go. Just using a single disk is not a good thing...



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