Re: RAID5 -> RAID6 conversion, please help

[Dylan Distasio <interzone@gmail.com>]

Hi Neil-

Just out of curiosity, how does mdadm decide which layout to use on a
reshape from RAID5->6.  I converted two of my RAID5s on different
boxes running the same OS awhile ago, and was not aware of the
different possibilities.   When I check now, one of them was converted
with the Q block all on the last disk, and the other appears
normalized.  I'm relatively confident I ran exactly the same command
on both to reshape them within a short time of one another.

Here are the current details of the two arrays:

dylan@terrordome:~$ sudo mdadm -D /dev/md0
/dev/md0:
        Version : 0.90
  Creation Time : Tue Mar  3 23:41:24 2009
     Raid Level : raid6
     Array Size : 5860559616 (5589.07 GiB 6001.21 GB)
  Used Dev Size : 976759936 (931.51 GiB 1000.20 GB)
   Raid Devices : 8
  Total Devices : 8
Preferred Minor : 0
    Persistence : Superblock is persistent

  Intent Bitmap : Internal

    Update Time : Tue May 10 20:06:42 2011
          State : active
 Active Devices : 8
Working Devices : 8
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric-6
     Chunk Size : 64K

           UUID : 4891e7c1:5d7ec244:a9bd8edb:
d35467d0 (local to host terrordome)
         Events : 0.743956

    Number   Major   Minor   RaidDevice State
       0       8       33        0      active sync   /dev/sdc1
       1       8       49        1      active sync   /dev/sdd1
       2       8       97        2      active sync   /dev/sdg1
       3       8      113        3      active sync   /dev/sdh1
       4       8       17        4      active sync   /dev/sdb1
       5       8       65        5      active sync   /dev/sde1
       6       8      241        6      active sync   /dev/sdp1
       7      65       17        7      active sync   /dev/sdr1
dylan@terrordome:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 10.04.1 LTS
Release:        10.04
Codename:       lucid


dylan@rapture:~$ sudo mdadm -D /dev/md0

/dev/md0:
        Version : 0.90
  Creation Time : Sat Jun  7 02:54:05 2008
     Raid Level : raid6
     Array Size : 2194342080 (2092.69 GiB 2247.01 GB)
  Used Dev Size : 731447360 (697.56 GiB 749.00 GB)
   Raid Devices : 5
  Total Devices : 5
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Tue May 10 20:19:13 2011
          State : clean
 Active Devices : 5
Working Devices : 5
 Failed Devices : 0
  Spare Devices : 0

         Layout : left-symmetric
     Chunk Size : 64K

           UUID : 83b4a7df:1d05f5fd:e368bf24:bd0fce41
         Events : 0.723556

    Number   Major   Minor   RaidDevice State
       0       8       18        0      active sync   /dev/sdb2
       1       8       34        1      active sync   /dev/sdc2
       2       8        2        2      active sync   /dev/sda2
       3       8       66        3      active sync   /dev/sde2
       4       8       82        4      active sync   /dev/sdf2

dylan@rapture:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 10.04.1 LTS
Release:        10.04
Codename:       lucid

On Tue, May 10, 2011 at 8:21 PM, NeilBrown <neilb@suse.de> wrote:
>
> On Wed, 11 May 2011 09:39:27 +1000 Steven Haigh <netwiz@crc.id.au> wrote:
>
> > On 11/05/2011 9:31 AM, NeilBrown wrote:
> > > When it finished you will have a perfectly functional RAID6 array with full
> > > redundancy.  It might perform slightly differently to a standard layout -
> > > I've never performed any measurements to see how differently.
> > >
> > > If you want to (after the recovery completes) you could convert to a regular
> > > RAID6 with
> > >    mdadm -G /dev/md0 --layout=normalise   --backup=/some/file/on/a/different/device
> > >
> > > but you probably don't have to.
> > >
> >
> > This makes me wonder. How can one tell if the layout is 'normal' or with
> > Q blocks on a single device?
> >
> > I recently changed my array from RAID5->6. Mine created a backup file
> > and took just under 40 hours for 4 x 1Tb devices. I assume that this
> > means that data was reorganised to the standard RAID6 style? The
> > conversion was done at about 4-6Mb/sec.
>
> Probably.
>
> What is the 'layout' reported by "mdadm -D"?
> If it ends -6, then it is a RAID5 layout with the Q block all on the last
> disk.
> If not, then it is already normalised.
>
> >
> > Is there any effect on doing a --layout=normalise if the above happened?
> >
> Probably not.
>
> NeilBrown
>
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