From mboxrd@z Thu Jan  1 00:00:00 1970
From: Goswin von Brederlow <goswin-v-b@web.de>
Subject: Re: mismatch_cnt again
Date: Sat, 14 Nov 2009 20:04:37 +0100
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In-Reply-To: <yq1my2ql56a.fsf@sermon.lab.mkp.net> (Martin K. Petersen's
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To: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Bill Davidsen <davidsen@tmr.com>, Peter Rabbitson <rabbit+list@rabbit.us>, NeilBrown <neilb@suse.de>, Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>, Goswin von Brederlow <goswin-v-b@web.de>, Doug Ledford <dledford@redhat.com>, Michael Evans <mjevans1983@gmail.com>, Eyal Lebedinsky <eyal@eyal.emu.id.au>, linux-raid list <linux-raid@vger.kernel.org>
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"Martin K. Petersen" <martin.petersen@oracle.com> writes:

>>>>>> "Bill" == Bill Davidsen <davidsen@tmr.com> writes:
>
>>> FWIW, XFS and btrfs both use the page writeback bit correctly and
>>> never change a page while it is undergoing I/O.
>>> 
>>> 
> Bill> That's necessary but not sufficient. To be done correctly it must
> Bill> be protected by md as well. This is because arrays are used
> Bill> without a filesystem by some applications, such as swap and
> Bill> database, to name the most common cases. 
>
> I agree that making MD RAID1 do a copy would be a quick fix.  But I
> don't see any reason to encourage what is essentially sloppy behavior at
> the top of the stack.  And then what if you stack MD/DM devices?  Do
> each layer do a copy?  I think that gets murky pretty quickly.

Maybe as a quick debug the raid layer should make the page read-only
and then watch what fails to write to it.

> I'd much rather fix the cases where the top layers are broken.  And as I
> said there are several people working on this spurred by my work on the
> data integrity extensions.
>
> FWIW, databases on raw disk have gone out of fashion.  But it is true
> that applications that do direct I/O need to avoid updating buffers in
> flight.

Maybe a flag somewhere saying if the data is safe from writes or
not. Default would be unsafe and md copies. A filesystem that works
"right" sets the safe flag as would md after copying. That way
anything lower in the stack (like another md) has the flag set.

MfG
        Goswin