Re: Fw: Why does one get mismatches?

[Goswin von Brederlow <goswin-v-b@web.de>]

Jon Hardcastle <jd_hardcastle@yahoo.com> writes:

> --- On Fri, 22/1/10, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>
>> From: Goswin von Brederlow <goswin-v-b@web.de>
>> Subject: Re: Fw: Why does one get mismatches?
>> To: Jon@eHardcastle.com
>> Cc: linux-raid@vger.kernel.org
>> Date: Friday, 22 January, 2010, 18:13
>> Jon Hardcastle <jd_hardcastle@yahoo.com>
>> writes:
>> 
>> > --- On Tue, 19/1/10, Jon Hardcastle <jd_hardcastle@yahoo.com>
>> wrote:
>> >
>> >> From: Jon Hardcastle <jd_hardcastle@yahoo.com>
>> >> Subject: Why does one get mismatches?
>> >> To: linux-raid@vger.kernel.org
>> >> Date: Tuesday, 19 January, 2010, 10:04
>> >> Hi,
>> >> 
>> >> I kicked off a check/repair cycle on my machine
>> after i
>> >> moved the phyiscal ordering of my drives around
>> and I am now
>> >> on my second check/repair cycle and it has kept
>> finding
>> >> mismatches.
>> >> 
>> >> Is it correct that the mismatch value after a
>> repair was
>> >> needed should equal the value present after a
>> check? What if
>> >> it doesn't? What does it mean if another check
>> STILL reveals
>> >> mismatches?
>> >> 
>> >> I had something similar after i reshaped from raid
>> 5 to 6 i
>> >> had to run check/repair/check/repair several times
>> before i
>> >> got my 0.
>> >> 
>> >> 
>> >
>> > Guys,
>> >
>> > Anyone got any suggestions here? I am now on my ~5
>> check/repair and after a reboot the first check is still
>> returning 8.
>> >
>> > All i have done is move the drives around. It is the
>> same controllers/cables/etc 
>> >
>> > I really dont like the seeming random nature of what
>> can/does/has caused the mismatches?
>> 
>> There is some unknown corruption going on with raid1 that
>> causes
>> mismatches but it is believed that it will never occur on
>> any used
>> block. Swapping is a likely cause.
>> 
>> Any swap device on the raid? Try turning that off.
>> If that doesn't help try umounting filesystems or
>> remounting RO.
>> 
>> MfG
>>         Goswin
>
> Hello, my usual savior Goswin!
>
> The deal is it is a 7 drive raid 6 array. it has LVM on it and is not used for swapping. I have umounted all LV's and still got mismatches, i run smartctl --test=long on all drives - nothing. I have now dismantled the array and am 3/4 the way through 'badblocks -svn' on each of the component drive. I have a hunch that it may be a dodgy SATA cable but have no evidence. No errors in log, nothing on dmesg.
>
> Is there any way to get more information? I am starting to think this is more happened since i changed from raid 5 to 6..... which i did < 1 month ago.
>
> The only lead i have is that whilst doing the bad blocks 1 drive ran at ~10~15MB/s whereas the rest are going at ~30 i have another identical model drive coming up so i will see if that one is slow too. But the lack of logging info is not helpful and worrying! and the prospect of silent corruption a big worry!

You did run a repair pass and not just repeated check passes, right?
Check itself only counts the mismatches but does not correct them.
If the raid is unused (vgchange -a n) and you do first repair and then
check then that definetly should not find any mismatches.

MfG

        Goswin
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