Re: reshape raid5 to raid6

[Michael Ole Olsen <gnu@gmx.net>]

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Hello I just compiled the 2.6.30 kernel from kernel.org
I was running 29.3 and 2.6.7.2 mdadm (Nov 08) Debian lenny, but kernel from
kernel.org.

I added the new device with echoing into /sys as you described

mfs:/home/michael# mdadm --zero-superblock /dev/sdc (It was an old disk from
the raid that was taken out long ago and not used - array was not degraded,
so I can just zero it)

mfs:/home/michael# mdadm --add /dev/md0 /dev/sdc

mfs:/home/michael# cat /proc/mdstat 
Personalities : [raid6] [raid5] [raid4] 
md0 : active raid6 sdc[10] sdi[0] sda[8] sdj[7] sdb[6] sde[5] sdg[4] sdd[3]
sdh[2] sdf[1]
      11721107968 blocks level 6, 64k chunk, algorithm 18 [10/9] [UUUUUUUUU_]
            [>....................]  recovery =  0.9% (14365824/1465138496)
    finish=414.1min speed=58382K/sec

the raid6 seems to be working, at least proc says so.

after stopping my reads and writes to the disk the reshape went 
from approximately 1000kB/s to 55000kB/s (i think i was only reading a bit from it)

440 minutes till reshape, so seems to go just fine.

I will try to re-stripe (and of course backup data first) with the new mdadm 
when it comes out, looking forward to it, thanks

/Michael Ole Olsen


NeilBrown schrieb am Wednesday, den 24. June 2009:

> On Wed, June 24, 2009 8:27 pm, Michael Ole Olsen wrote:
> > Is it possible to reshape my /dev/md0 raid5 into raid6?
> 
> If you are are using Linux 2.6.30, then you can
> 
>   echo raid6 > /sys/block/md0/md/level
> 
> and it will instantly be sort-of-raid6.
> It is exactly like raid6 except that the Q blocks are all one
> the one drive, and drive that previously didn't exist.
> If you have a spare, it will start building the Q blocks
> on that drive and when it finishes you will have true raid6
> redundancy, though possibly a little less than raid6 performance,
> as a real raid6 has the Q block distributed.
> 
> When mdadm-3.1 is released, you will be able to tell the raid6
> to re-stripe with a more traditional layout.  This will take quite
> a while, but you can continue to use the array (though a bit more
> slowly) will it progresses.
> Of course you don't need to do that step if you don't want to.
> 
> 
> 
> >
> > I found the following recipe in the docs:
> > mdadm --create /dev/md0 -l6 -n4 /dev/sdb1 /dev/sdc1 /dev/sdd1 missing
> >
> > I have 9 disks in my raid5, would i then do:
> > [1] mdadm --create /dev/md0 -l6 -n10 /dev/sda /dev/sdb ... missing
> 
> If you did this, an the drives contained raid5 data, then that data
> would not be available on /dev/md0.
> 
> NeilBrown
> 
> 
> >
> > i.e. only the first 9 drives and then hot add a new device to get it to
> > reshape and add the extra parity to all drives?
> >
> > mdadm --add /dev/md0 /dev/sdk ?
> >
> > I would guess  command [1] would give problems as md0 is an active array
> > will it work if i --stop the array first so that i can reshape to raid6?
> >
> > I couldn't really find much information on this, some say it is impossible
> > without recreating the array? (So i ask here first before i try, as I dont
> > want to risk my data, even though I have backups of the important
> > data)
> >
> > Any advice would be nice
> >
> > /Michael Ole Olsen
> >
> 
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