Stan Hoeppner <stan <at> hardwarefreak.com> writes:
>
> On 3/27/2014 10:23 AM, Bernd Schubert wrote:
> ...
> >> commit 508a7f16b242d6c3353e15aab46ac8ca8dc7cd08
> >> Author: NeilBrown <neilb <at> 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
Yes, it's not fully explain the difference. Here's another test with mdadm
v3.2.6 (which already have the 508a7f16b242d6c3353e15aab46ac8ca8dc7cd08
commit) in CentOS 6.5.
Created a RAID 5 array with 5 1GiB loop devices, the data_offset of this new
array is only 1024 sectors (0.5M).
# MAKEDEV /dev/loop
# for hdd in {1..5}; do truncate -s 1G hdd5$hdd; losetup /dev/loop5$hdd
hdd5$hdd; done
# mdadm -C /dev/md5 -l5 -n4 -x1 /dev/loop5[1-5]
# mdadm --version
mdadm - v3.2.6 - 25th October 2012
# mdadm --examine /dev/loop51
/dev/loop51:
Magic : a92b4efc
Version : 1.2
Feature Map : 0x0
Array UUID : 5fb7e243:01760f20:c773f451:09928fa0
Name : RecordFileServer:5 (local to host RecordFileServer)
Creation Time : Fri Mar 28 10:14:36 2014
Raid Level : raid5
Raid Devices : 4
Avail Dev Size : 2096128 (1023.67 MiB 1073.22 MB)
Array Size : 3142656 (3.00 GiB 3.22 GB)
Used Dev Size : 2095104 (1023.17 MiB 1072.69 MB)
Data Offset : 1024 sectors
Super Offset : 8 sectors
State : clean
Device UUID : d662c632:b2aafbac:406c84a6:ae519102
Update Time : Fri Mar 28 10:15:07 2014
Checksum : ab92e515 - correct
Events : 20
Layout : left-symmetric
Chunk Size : 512K
Device Role : Active device 0
Array State : AAAA ('A' == active, '.' == missing)
# mdadm -D /dev/md5
/dev/md5:
Version : 1.2
Creation Time : Fri Mar 28 10:14:36 2014
Raid Level : raid5
Array Size : 3142656 (3.00 GiB 3.22 GB)
Used Dev Size : 1047552 (1023.17 MiB 1072.69 MB)
Raid Devices : 4
Total Devices : 5
Persistence : Superblock is persistent
Update Time : Fri Mar 28 10:14:56 2014
State : clean
Active Devices : 4
Working Devices : 5
Failed Devices : 0
Spare Devices : 1
Layout : left-symmetric
Chunk Size : 512K
Name : RecordFileServer:5 (local to host RecordFileServer)
UUID : 5fb7e243:01760f20:c773f451:09928fa0
Events : 20
Number Major Minor RaidDevice State
0 7 51 0 active sync /dev/loop51
1 7 52 1 active sync /dev/loop52
2 7 53 2 active sync /dev/loop53
5 7 54 3 active sync /dev/loop54
4 7 55 - spare /dev/loop55
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