From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stan Hoeppner <stan@hardwarefreak.com>
Subject: Re: Array created by mdadm 3.2 & 3.3 have different array size, why?
Date: Wed, 26 Mar 2014 16:14:47 -0500
Message-ID: <53334347.4050108@hardwarefreak.com>
References: <tencent_66C345191B9B49B92956F243@qq.com> <alpine.DEB.2.02.1403261937480.747@uplift.swm.pp.se> <loom.20140326T205013-837@post.gmane.org>
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To: Tide <lovetide@qq.com>, linux-raid@vger.kernel.org
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On 3/26/2014 3:00 PM, Tide wrote:
...
> =================
> Array 2 (RAID 6):
> =================
> # mdadm --examine /dev/sdb1
...
>      Raid Level : raid6
>    Raid Devices : 5
...
>   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.

Cheers,

Stan