On Sun, 12 Jan 2014 23:37:57 -0500 Richard Michael
<rmichael@xxxxxxxxxxxxxxxx> wrote:
> Hello list,
>
> I grew a RAID0 by one-disk, and it re-shaped via RAID4 as expected.
>
> However, the component superblocks still RAID4, while /proc/mdstat,
> /sys/block/md0/md/level and "mdadm -D" all indicate RAID0.
>
> I am reluctant to stop the array, in case auto-assemble can't put it
> back together. (I suppose I could create a new array, but I'd want to
> be quite confident about the layout of the disks.)
>
>
> Is this a bug? Should/can I re-write the superblock(s)?
>
>
> # cat /proc/mdstat
> Personalities : [raid0] [raid1] [raid6] [raid5] [raid4]
> md0 : active raid0 sdc1[2] sdd1[0]
> 5860268032 blocks super 1.2 512k chunks
>
> # cat /sys/block/md0/md/level
> raid0
>
> # mdadm -D /dev/md0
> /dev/md0:
> Version : 1.2
> Creation Time : Fri Jan 10 13:02:25 2014
> Raid Level : raid0
> Array Size : 5860268032 (5588.79 GiB 6000.91 GB)
> Raid Devices : 2
> Total Devices : 2
> Persistence : Superblock is persistent
>
> Update Time : Sun Jan 12 20:08:53 2014
> State : clean
> Active Devices : 2
> Working Devices : 2
> Failed Devices : 0
> Spare Devices : 0
>
> Chunk Size : 512K
>
> Number Major Minor RaidDevice State
> 0 8 49 0 active sync /dev/sdd1
> 2 8 33 1 active sync /dev/sdc1
>
>
>
> But,
>
>
> # mdadm -E /dev/sd[cd]1
> /dev/sdc1:
> Magic : a92b4efc
> Version : 1.2
> Feature Map : 0x0
> Array UUID : 8f51352a:610d0ecd:a1e28ddd:86c8586c
> Name : anvil.localdomain:0 (local to host anvil.localdomain)
> Creation Time : Fri Jan 10 13:02:25 2014
> Raid Level : raid4
> Raid Devices : 3
>
> Avail Dev Size : 5860268943 (2794.39 GiB 3000.46 GB)
> Array Size : 5860268032 (5588.79 GiB 6000.91 GB)
> Used Dev Size : 5860268032 (2794.39 GiB 3000.46 GB)
> Data Offset : 260096 sectors
> Super Offset : 8 sectors
> Unused Space : before=260008 sectors, after=2959 sectors
> State : clean
> Device UUID : ad6e6c88:0f897bc1:1f6ec909:f599bc01
>
> Update Time : Sun Jan 12 20:08:53 2014
> Bad Block Log : 512 entries available at offset 72 sectors
> Checksum : 1388a7b - correct
> Events : 9451
>
> Chunk Size : 512K
>
> Device Role : Active device 1
> Array State : AA. ('A' == active, '.' == missing, 'R' == replacing)
> /dev/sdd1:
> Magic : a92b4efc
> Version : 1.2
> Feature Map : 0x0
> Array UUID : 8f51352a:610d0ecd:a1e28ddd:86c8586c
> Name : anvil.localdomain:0 (local to host anvil.localdomain)
> Creation Time : Fri Jan 10 13:02:25 2014
> Raid Level : raid4
> Raid Devices : 3
>
> Avail Dev Size : 5860268032 (2794.39 GiB 3000.46 GB)
> Array Size : 5860268032 (5588.79 GiB 6000.91 GB)
> Data Offset : 260096 sectors
> Super Offset : 8 sectors
> Unused Space : before=260008 sectors, after=2959 sectors
> State : clean
> Device UUID : b3cda274:547919b1:4e026228:0a4981e7
>
> Update Time : Sun Jan 12 20:08:53 2014
> Bad Block Log : 512 entries available at offset 72 sectors
> Checksum : e16a1979 - correct
> Events : 9451
>
> Chunk Size : 512K
>
> Device Role : Active device 0
> Array State : AA. ('A' == active, '.' == missing, 'R' == replacing)
>
>
>
> Somewhat aside, I grew the array with:
>
> "mdadm --grow /dev/md0 --raid-devices=2 --add /dev/sdc1"
That is the correct command.
>
> I suspect I should not have used "--add". Looking at the superblock,
> there is a 3rd unknown device, which I did not intend to add.
>
> Did I convince mdadm to add two devices at the same time, sdc1 *and* a
> missing device? (This surprises me a bit, in the sense that
> --raid-devices=2 would pertain to the added devices, rather than the
> total devices in the array.)
>
> Or, perhaps mdadm add a "dummy" device as part of the temporary RAID4
> conversion?
Exactly. The RAID4 had 1 more device than the RAID0. What is what you are
seeing.
I'm a bit confused ... did you grow this from a 1-device RAID0 to a 2-device
RAID0? That seems like an odd thing to do, but it should certainly work.
This should work and I think I've tested it. However looking at the code I
cannot see how it ever would have done. I cannot see anything that would
write out the new metadata to the RAID0 after the reshape completes.
Normally md will never write to the metadata of a RAID0 so it would need
special handling which doesn't seem to be there.
I just tried testing it on the current mainline kernel and it crashes :-(
So it looks like I need to do some fixing here.
Your array should continue to work. If you reboot, it will be assembled as a
RAID4 with the parity disk missing. This will work perfectly but may not be
as fast as RAID0. You can "mdadm --grow /dev/md0 --level=0" to convert it to
RAID0 though it probably won't cause the metadata to be updated.
Thanks for the report.
NeilBrown
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