Re: Growing a RAID array

[NeilBrown <neilb@suse.de>]

On Sat, 14 May 2011 02:17:27 -0500 "Leslie Rhorer" <lrhorer@satx.rr.com>
wrote:

> > -----Original Message-----
> > From: NeilBrown [mailto:neilb@suse.de]
> > Sent: Thursday, May 12, 2011 8:23 PM
> > To: lrhorer@satx.rr.com
> > Cc: linux-raid@vger.kernel.org
> > Subject: Re: Growing a RAID array
> > 
> > On Thu, 12 May 2011 20:02:01 -0500 "Leslie Rhorer" <lrhorer@satx.rr.com>
> > wrote:
> > 
> > >
> > > 	I am shortly going to be growing a 10 drive RAID6 array with two
> > > additional drives.  Can this be done in a single grow operation by
> > setting
> > > n=12, or do I have to grow it 1 drive at a time?
> > >
> > 
> > Experiment with loop-back devices created on modest sized files.
> 
> 	OK, I tried this, but for some reason the re-shape is failing, even
> with 1 "disk".  I've grown arrays under mdadm any number of times, and I'm
> not sure why this is failing.  I created 8 loop devices of 10M each.  I then
> created a RAID6 array with 6 members and 2 spares.  When I try to add one or
> both drives, I get an error:
> 
> RAID-Server:/RAID# mdadm -G /dev/md6 -n 8 --backup-file=/RAID/raid6.bak
> mdadm: Need to backup 12288K of critical section..
> mdadm: /dev/md6: Something wrong - reshape aborted
> 
> 	Here is the reported layout:
> 
> RAID-Server:/RAID# mdadm -D /dev/md6
> /dev/md6:
>         Version : 1.2
>   Creation Time : Fri May 13 23:15:39 2011
>      Raid Level : raid6
>      Array Size : 36864 (36.01 MiB 37.75 MB)
>   Used Dev Size : 9216 (9.00 MiB 9.44 MB)
>    Raid Devices : 6
>   Total Devices : 8
>     Persistence : Superblock is persistent
> 
>   Intent Bitmap : Internal
> 
>     Update Time : Fri May 13 23:23:04 2011
>           State : active
>  Active Devices : 6
> Working Devices : 8
>  Failed Devices : 0
>   Spare Devices : 2
> 
>          Layout : left-symmetric
>      Chunk Size : 1024K

10Meg devices with a 1Meg chunk size means just 10 stripes.

To grow from 4 to 5 data disks it calculates that it needs to back up the
first 20 stripes (I think).  It sees this is more than half the device and
gets confused and assumes something is wrong.

Try with 4k chunks, or 100Meg devices.

NeilBrown


> 
>            Name : RAID-Server:6  (local to host RAID-Server)
>            UUID : 332f387c:6c301089:22904f6c:8089b9b4
>          Events : 30
> 
>     Number   Major   Minor   RaidDevice State
>        0       7        0        0      active sync   /dev/loop0
>        1       7        1        1      active sync   /dev/loop1
>        2       7        2        2      active sync   /dev/loop2
>        3       7        3        3      active sync   /dev/loop3
>        4       7        4        4      active sync   /dev/loop4
>        5       7        5        5      active sync   /dev/loop5
> 
>        6       7        6        -      spare   /dev/loop6
>        7       7        7        -      spare   /dev/loop7
> 
> 
> 	The mdadm version is 3.1.4 under kernel 2.6.32-5-amd64
> 
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