From mboxrd@z Thu Jan  1 00:00:00 1970
From: Goswin von Brederlow <goswin-v-b@web.de>
Subject: Re: Why does one get mismatches?
Date: Sun, 21 Feb 2010 12:13:02 +0100
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In-Reply-To: <4B7FC3A7.10104@shiftmail.org> (asdo@shiftmail.org's message of
	"Sat, 20 Feb 2010 12:12:39 +0100")
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To: Asdo <asdo@shiftmail.org>
Cc: Goswin von Brederlow <goswin-v-b@web.de>, linux-raid@vger.kernel.org
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Asdo <asdo@shiftmail.org> writes:

> Goswin von Brederlow wrote:
>> The check is usualy done with the filesystem mounted and in use. So one
>> case would be that the block got written, changed and then checked
>> before the FS decided to flush the dirty block again.
>>
>> The other scenario suggested in the past is that the block was written,
>> changed and then the file deleted, making the block unused,
> This is not enough to cause the problem if I understand correctly, it
> also needs to change value at this point, right?
> So how can it change value... is the same buffer used for another block?

open tempfile
write tempfile
raid1 starts to commit the block
write some more changing the block
 raid1 writes the 2nd copy of the block
delete file
fs never recommits the dirty page

Personally I don't really buy that scenario. At least not in the
frequency mismatches occur.

>> before it
>> got flushed again. The filesystem then sees no need to write a dirty but
>> unused block so it never gets rewritten. It never gets read either so
>> that  is safe.
>>

MfG
        Goswin