check vs repair -- mismatch count goes up in repair, zeros in check?

I'm a bit confused about check vs repair.

When I run repair, mismatches are initially zero'd at the start and
then add up as it runs through, so that at the end there is a positive
mismatch count. Running check after that zero's them out.

I thought check would run through the array and identify mismatches,
but not repair them, while repair would do the repair step as well.
But the exhibited behavior seems to indicate the opposite.

Does the mismatch count show the current number of identified
mismatches? Or what is it really counting?

See http://pastthe.endoftheinternet.org/munin/home/ubuntunas/ind ex.html#munin
for an example (page up from the munin graphs--the three graphs
immediately above track the array.


/dev/md0:
Version : 1.2
Creation Time : Sat Apr 30 14:29:07 2011
Raid Level : raid6
Array Size : 5857609728 (5586.25 GiB 5998.19 GB)
Used Dev Size : 1952536576 (1862.08 GiB 1999.40 GB)
Raid Devices : 5
Total Devices : 6
Persistence : Superblock is persistent

Update Time : Sat Jun 18 09:08:03 2011
State : clean
Active Devices : 5
Working Devices : 6
Failed Devices : 0
Spare Devices : 1

Layout : left-symmetric
Chunk Size : 512K

Name : ubuntunas:0 (local to host ubuntunas)
UUID : 9916bc94:a245d51b:ecca3480:7c612ed5
Events : 127

Number Major Minor RaidDevice State
0 8 2 0 active sync /dev/sda2
1 8 18 1 active sync /dev/sdb2
2 8 34 2 active sync /dev/sdc2
3 8 50 3 active sync /dev/sdd2
4 8 66 4 active sync /dev/sde2

5 8 82 - spare /dev/sdf2
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Re: check vs repair -- mismatch count goes up in repair, zeros incheck?

On Sat, 18 Jun 2011 09:10:01 -0700 Rory Jaffe wrote:

> I'm a bit confused about check vs repair.
>
> When I run repair, mismatches are initially zero'd at the start and
> then add up as it runs through, so that at the end there is a positive
> mismatch count. Running check after that zero's them out.
>
> I thought check would run through the array and identify mismatches,
> but not repair them, while repair would do the repair step as well.
> But the exhibited behavior seems to indicate the opposite.

Does it? Why do you think that 'check' found no mismatches? Because
'repair' had already repaired them!

Both 'check' and 'repair' report how many mismatches were found. 'check'
doesn't repair them. 'repair' does.

>
> Does the mismatch count show the current number of identified
> mismatches? Or what is it really counting?

It is really counting sectors that might not match.
i.e. it performs the check is section larger than one sector. When it finds
a mismatch it just adds the number of sectors in the section to the count.

NeilBrown


>
> See http://pastthe.endoftheinternet.org/munin/home/ubuntunas/ind ex.html#munin
> for an example (page up from the munin graphs--the three graphs
> immediately above track the array.
>
>
> /dev/md0:
> Version : 1.2
> Creation Time : Sat Apr 30 14:29:07 2011
> Raid Level : raid6
> Array Size : 5857609728 (5586.25 GiB 5998.19 GB)
> Used Dev Size : 1952536576 (1862.08 GiB 1999.40 GB)
> Raid Devices : 5
> Total Devices : 6
> Persistence : Superblock is persistent
>
> Update Time : Sat Jun 18 09:08:03 2011
> State : clean
> Active Devices : 5
> Working Devices : 6
> Failed Devices : 0
> Spare Devices : 1
>
> Layout : left-symmetric
> Chunk Size : 512K
>
> Name : ubuntunas:0 (local to host ubuntunas)
> UUID : 9916bc94:a245d51b:ecca3480:7c612ed5
> Events : 127
>
> Number Major Minor RaidDevice State
> 0 8 2 0 active sync /dev/sda2
> 1 8 18 1 active sync /dev/sdb2
> 2 8 34 2 active sync /dev/sdc2
> 3 8 50 3 active sync /dev/sdd2
> 4 8 66 4 active sync /dev/sde2
>
> 5 8 82 - spare /dev/sdf2
> --
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html

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