Stan Hoeppner <stan <at> hardwarefreak.com> writes:

On 3/26/2014 3:00 PM, Tide wrote:
...

quoted

=================
Array 2 (RAID 6):
=================
# mdadm --examine /dev/sdb1

...

quoted

     Raid Level : raid6
   Raid Devices : 5

...

quoted

  Bad Block Log : 512 entries available at offset 72 sectors

The RAID6 array has sectors on each drive reserved for bad block
reassignment.  The RAID5 array does not.

This is the answer to your mystery.

"The RAID6/RAID5 array", do you mean my RAID array (just this instance), or
you mean "all RAID6/RAID5 arrays created by mdadm" ?

I also did created another RAID 5 & RAID 6 testing arrays using loopback
devices in both CentOS 6.5 and Fedora 20, and found out: array size of RAID
5 & RAID 6 are identical in same OS (in CentOS or in Fedora), but they are
different between OSes. That's what I still feel mysterious.

So is it different behaviour between mdadm v3.2 and v3.3, or it's just the 
compilation parameters of mdadm are different between CentOS 6.x and Fedora
19/20?

Testing script:
MAKEDEV /dev/loop; truncate -s 10M hdd5{1..5} hdd6{1..5}; for hdd in {1..5};
do losetup /dev/loop5$hdd hdd5$hdd; losetup /dev/loop6$hdd hdd6$hdd; done;
mdadm -C /dev/md5 -l 5 -n 4 -x 1 /dev/loop5{1..5}; mdadm -C /dev/md6 -l 6 -n
5 /dev/loop6{1..5}; mdadm -D /dev/md5; mdadm -D /dev/md6

Cheers,

Stan

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