linux-ext4 - Re: [patch] document flash/RAID dangers

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Date:	Wed, 26 Aug 2009 06:44:42 -0700 (PDT)
From:	david@...g.hm
To:	Ric Wheeler <rwheeler@...hat.com>
cc:	Theodore Tso <tytso@....edu>, Pavel Machek <pavel@....cz>,
	Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	Rob Landley <rob@...dley.net>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: [patch] document flash/RAID dangers

On Wed, 26 Aug 2009, Ric Wheeler wrote:

> On 08/26/2009 08:40 AM, Theodore Tso wrote:
>> On Wed, Aug 26, 2009 at 07:58:40AM -0400, Ric Wheeler wrote:
>>>> 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?
>> 
>> Well, the softwar raid layer could be improved so that it implements
>> scrubbing by default (i.e., have the md package install a cron job to
>> implement a periodict scrub pass automatically).  The MD code could
>> also regularly check to make sure the hot spare is OK; the other
>> possibility is that hot spare, which hadn't been used in a long time,
>> had silently failed.
>
> Actually, MD does this scan already (not automatically, but you can set up a 
> simple cron job to kick off a periodic "check"). It is a delicate balance to 
> get the frequency of the scrubbing correct.

debian defaults to doing this once a month (first sunday of each month), 
on some of my systems this scrub takes almost a week to complete.

David Lang
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