Re: Array created by mdadm 3.2 & 3.3 have different array size, why?

[Stan Hoeppner <stan@hardwarefreak.com>]

On 3/27/2014 10:23 AM, Bernd Schubert wrote:
> On 03/26/2014 10:14 PM, Stan Hoeppner wrote:
>> 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.
> 
> Commits and code do not confirm this assumption.
> 
>>         __u16   bblog_size;     /* number of sectors reserved for
>> badblocklist */
> 
> ...
> 
>>                 printf("  Bad Block Log : %d entries available at
>> offset %ld sectors",
>>                        __le16_to_cpu(sb->bblog_size)*512/8,
> 
> 
> So 512 bad-block-log entries only need 8 sectors and that would still
> fit into the data offset of 2048 bytes (array 1). The
> write-intent-bitmap is also not that big. But this commit log gives the
> correct answer
> 
>> commit 508a7f16b242d6c3353e15aab46ac8ca8dc7cd08
>> Author: NeilBrown <neilb@suse.de>
>> Date:   Wed Apr 4 14:00:40 2012 +1000
>>
>>     super1: leave more space in front of data by default.
>>
>>     The kernel is growing the ability to avoid the need for a
>>     backup file during reshape by being able to change the data offset.
>>
>>     For this to be useful we need plenty of free space before the
>>     data so the data offset can be reduced.
>>
>>     So for v1.1 and v1.2 metadata make the default data_offset much
>>     larger.  Aim for 128Meg, but keep a power of 2 and don't use more
>>     than 0.1% of each device.
>>
>>     Don't change v1.0 as that is used when the data_offset is required to
>>     be zero.

This is a good match because the discrepancy on his RAID6 is pretty
close to 128MB per drive.  However, both his RAID5 and RAID6 arrays are
metadata 1.2.  So this commit alone may not fully explain the capacity
difference he's seeing between RAID5 and RAID6.  Or is this commit RAID6
specific?  I don't see that in the comments above.

Cheers,

Stan