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Message-ID: <9a87484905091117222d318f4@mail.gmail.com>
Date: Mon, 12 Sep 2005 02:22:08 +0200
From: Jesper Juhl <jesper.juhl@gmail.com>
Reply-To: jesper.juhl@gmail.com
To: Neil Brown <neilb@cse.unsw.edu.au>
Subject: Re: read-from-all-disks support for RAID1?
Cc: Lennert Buytenhek <buytenh@wantstofly.org>, linux-kernel@vger.kernel.org
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	 <17188.49961.268818.355923@cse.unsw.edu.au>
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On 9/12/05, Neil Brown <neilb@cse.unsw.edu.au> wrote:
> On Saturday September 10, buytenh@wantstofly.org wrote:
> > (please CC on replies)
> >
> > Hi!
> >
> > I recently had a case where one disk in a two-disk RAID1 array went
> > subtly bad, effectively refusing to write to certain sectors without
> > reporting an error.  Basically, parts of the disk went undetectably
> > read-only, causing file system corruption that wouldn't go away after
> > fsck, and all kinds of other fun.
> 
> That really isn't something that a drive should do.  If a write fails,
> you need to be told that it failed.  If anything else happens, maybe
> you should consider boycotting that manufacturer, or at least buying
> more expensive drives (do I guess right that there were fairly
> cheap??).
> 
> 
> >
> > Would it be hard/wise to add an option for RAID1 mode to read from all
> > devices on a read, and report an error to syslog or simply return an
> > I/O error if there is a mismatch?  (Or use majority voting and tell
> > people to use 3-disk RAID1 arrays from now on ;-)
> >
> 
> No, I don't think so.  The overhead would be substantial, so people
> would be very unlikely to use it.

There are situations where data integrity is far more important than speed.
On AIX I usually use the Mirror Write Consistency and Write Verify
options on my mirrored volumes that store data where integrity is more
important than speed.
I guess something like those options would also satisfy Lennert's
needs, but I don't know if it's currently possible with the Linux LVM
or elsewhere.

You can read a bit about the MWC and WV options in AIX at :
http://publib.boulder.ibm.com/infocenter/pseries/index.jsp?topic=/com.ibm.aix.doc/aixbman/prftungd/diskperf2.htm


-- 
Jesper Juhl <jesper.juhl@gmail.com>
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