Re: [patch] document flash/RAID dangers

From: david
Date: Wed Aug 26 2009 - 09:45:50 EST

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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