From: Neil Brown <neilb@suse.de>
To: Phil Karn <karn@ka9q.net>
Cc: linux-raid@vger.kernel.org
Subject: Re: RAID-5 implementation questions
Date: Fri, 3 Dec 2010 22:08:07 +1100 [thread overview]
Message-ID: <20101203220807.68ce9f58@notabene.brown> (raw)
In-Reply-To: <4CF8AF30.5070505@ka9q.net>
On Fri, 03 Dec 2010 00:49:52 -0800 Phil Karn <karn@ka9q.net> wrote:
> Are there any papers documenting the implementation of the Linux RAID
> subsystem? I'm interested in some of the details of how RAID-5 works.
I would suggest
man mdadm
and
man md
That should answer at least some of your questions.
Then try http://raid.wiki.kernel.org/
If you have further questions after that, please ask.
NeilBrown
>
> I've never seen a virgin disk drive from the factory that wasn't all
> 0's. Creating a RAID array on a set of such drives triggers an initial
> rebuild that simply writes lots zeroes on lots of zeroes. With disks now
> pushing past 2 TB, this can easily take half a day.
>
> Except for the admittedly somewhat useful side effect of scanning the
> disks for bad sectors, all this activity seems rather unnecessary. Is
> there a way to create a RAID-5 (or any other RAID level) array so that
> it will immediately come up without an initial rebuild?
>
> File systems generally don't read disk blocks that they haven't already
> written. So even when you build a RAID array from drives with old data,
> I can't see how skipping the initial rebuild can cause any real harm.
> The first write to any block causes the RAID system to initialize the
> parity in that stripe, thus making it possible to regenerate that block
> in case of a drive failure.
>
> During the initial rebuild of a RAID-5 array, /proc/mdstat suggests that
> the array is operating in degraded mode and the last drive in the array
> is being rebuilt. Is this true, i.e., are all the rebuild writes going
> to that last drive?
>
> How does a rebuilding RAID-5 array handle a read or write operation when
> it lands on the "broken" drive? Does it depend on whether the block is
> before or after the rebuild pointer?
>
> Thanks much,
>
> Phil
> --
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prev parent reply other threads:[~2010-12-03 11:08 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-12-03 8:49 RAID-5 implementation questions Phil Karn
2010-12-03 10:02 ` Mikael Abrahamsson
2010-12-03 12:02 ` Phil Karn
2010-12-03 11:08 ` Neil Brown [this message]
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