Re: Shrinking number of devices on a RAID-10 (near 2) array

[NeilBrown <neilb@suse.de>]

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On Mon, 25 Aug 2014 11:34:03 +0000 Andy Smith <andy@strugglers.net> wrote:

> Hi Neil,
> 
> On Mon, Aug 25, 2014 at 09:26:14PM +1000, NeilBrown wrote:
> > What does "mdadm --examine" on one of the devices report now that it is v1.0?
> > 
> > Particularly interested in the "Unused Space :" line.
> 
> /dev/sda3:
>           Magic : a92b4efc
>         Version : 1.0
>     Feature Map : 0x0
>      Array UUID : 3905b303:ca604b72:be5949c4:ab051b7a
>            Name : 2
>   Creation Time : Sun Jun  4 08:18:59 2006
>      Raid Level : raid10
>    Raid Devices : 6
> 
>  Avail Dev Size : 618726528 (295.03 GiB 316.79 GB)
>      Array Size : 928089792 (885.10 GiB 950.36 GB)
>    Super Offset : 618727392 sectors
>    Unused Space : before=0 sectors, after=864 sectors
>           State : active
>     Device UUID : e30176be:81a57e84:1f2aa206:9515150f
> 
>     Update Time : Sun Aug 24 14:29:15 2014
>        Checksum : bd599c72 - correct
>          Events : 1
> 
>          Layout : near=2
>      Chunk Size : 64K
> 
>    Device Role : Active device 5
>    Array State : AAAAAA ('A' == active, '.' == missing, 'R' == replacing)
> 
> All other devices report same (apart from UUID/checksum/role).

'after=864' should be plenty.

mdadm writes '128' to 'new_offset'
This is bigger than 'data_offset' (which must be zero, or it would have been
reported), mddev->dev_sectors is 618726528 while rdev->sectors is 618727392
(difference is 864), so 
		if (new_offset - rdev->data_offset
		    + mddev->dev_sectors > rdev->sectors)
cannot return 'true'.  Yet it seems that it does.

Ahh.. somewhere else returns -E2BIG:

	if (mddev->pers && mddev->persistent &&
	    !super_types[mddev->major_version]
	    .allow_new_offset(rdev, new_offset))
		return -E2BIG;

But super_1_allow_new_offset always succeeds if new_offset >
rdev->data_offset, so that cannot be the problem.


I know.  rdev->sectors isn't "Super Offset", it is only "Avail Dev Size".
You need to
  echo 618727392 > /sys/block/md2/md/dev-sda3/size
and same for the other devices.

I looks like I need to get mdadm to be more clever when it gets -E2BIG.
Can you check that the above works?

Thanks,
NeilBrown


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