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

[NeilBrown <neilb@suse.de>]

[-- Attachment #1: Type: text/plain, Size: 2578 bytes --]

On Sat, 23 Aug 2014 16:31:10 +0000 Andy Smith <andy@strugglers.net> wrote:

> Hi,
> 
> I am aware that for a very long time it was not possible to change
> the number of devices in an mdadm RAID-10. But then recently I am
> sure I saw threads here alluding to this being possible. e.g.:
> 
>     http://marc.info/?l=linux-raid&m=140768923829685&w=2
> 
> I have a 6 device RAID-10 near=2 array that I would like to shrink
> down to 4 devices.
> 
> I have compiled mdadm 3.3.2 and am using kernel 3.16.0.
> 
> $ sudo mdadm --detail /dev/md2
> /dev/md2:
>         Version : 0.90
>   Creation Time : Sun Jun  4 08:18:58 2006
>      Raid Level : raid10
>      Array Size : 471859200 (450.00 GiB 483.18 GB)
>   Used Dev Size : 309363520 (295.03 GiB 316.79 GB)
>    Raid Devices : 6
>   Total Devices : 6
> Preferred Minor : 2
>     Persistence : Superblock is persistent
> 
>     Update Time : Sat Aug 23 15:23:38 2014
>           State : active 
>  Active Devices : 6
> Working Devices : 6
>  Failed Devices : 0
>   Spare Devices : 0
> 
>          Layout : near=2
>      Chunk Size : 64K
> 
>            UUID : 3905b303:ca604b72:be5949c4:ab051b7a
>          Events : 0.312149991
> 
>     Number   Major   Minor   RaidDevice State
>        0       8       51        0      active sync   /dev/sdd3
>        1       8       67        1      active sync   /dev/sde3
>        2       8       83        2      active sync   /dev/sdf3
>        3       8       19        3      active sync   /dev/sdb3
>        4       8       35        4      active sync   /dev/sdc3
>        5       8        3        5      active sync   /dev/sda3
> 
> $ sudo ./mdadm --grow -n4 /dev/md2 --backup-file /var/tmp/mdadm.backup
> mdadm: Cannot set new_data_offset: RAID10 reshape not
>        supported on this kernel
> 
> Is that not yet possible, then?
> 
> (Each device is 320GB so it should all fit with only four of them)

raid10 reshape is only supported for v1.x metadata, and you have 0.90.
This is because it needs to move the data_offset and 0.90 cannot do that.

I see you have already adjusted the --array-size correctly - good.

You can achieve your result by:

 1/ stop the array and assemble with --update=metadata.  This will convert the
    array to v1.0
 2/ "mdadm /dev/md2 --grow --disks=4"  That should perform the reshape.
    You don't need a --backup-file with RAID10 ... or with other arrays if you
    have recent tools and enough space before or after.

I guess the error message could be better....

NeilBrown

[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 828 bytes --]