I just tried and miss-match count is zero. Interesting, is XOR engine
doing something wrong ? . Then I ran a test where I raw-write the file
to /dev/md0. Then did the raw-read for the same size. In this case XOR
matched as expected. Then I failed a drive using "mdadm -f /dev/md0
/dev/sda". Then I read the same size data again from /dev/md0. And
checksum matches too. 
What is this mean XOR engine is doing right thing. But "chec/repair"
test is not functioning properly with XOR-engine ?

Or is this something to do with  how the buffers are handled ? may they
are cached ?


-Marri



-----Original Message-----
From: goswin-v-b@web.de [mailto:goswin-v-b@web.de] 
Sent: Thursday, January 28, 2010 10:24 AM
To: Tirumala Reddy Marri
Cc: linux-raid@vger.kernel.org
Subject: Re: Why does one get mismatches?

"Tirumala Reddy Marri" [off-list ref] writes:

I have noticed that if I zero the /dev/md0 using "dd if=/dev/zero
of=/dev/md0 bs=4k count=64k". Then I run the "echo check >
/sys/block/md0/md/sync_action", no mismatch_cnt reported.  If I use
"if=/some/file" then I see miss match count to set to huge number.

I am testing with small RAID-5 size for quick testing.  How a reliable
is this test ? I am using HW accelerated XOR engine for RAID-5.

Have you tried without?

Maybe the XOR engine creates garbage.

MfG
        Goswin