Re: Growing 6 HDD RAID5 to 7 HDD RAID6

["Mathias Burén" <mathias.buren@gmail.com>]

On 13 April 2011 12:44, John Robinson <john.robinson@anonymous.org.uk> wrote:
> (Subject line amended by me :-)
>
> On 12/04/2011 17:56, Mathias Burén wrote:
> [...]
>>
>> I'm approaching over 6.5TB of data, and with an array this large I'd
>> like to migrate to RAID6 for a bit more safety. I'm just checking if I
>> understand this correctly, this is how to do it:
>>
>> * Add a HDD to the array as a hot spare:
>> mdadm --manage /dev/md0 --add /dev/sdh1
>>
>> * Migrate the array to RAID6:
>> mdadm --grow /dev/md0 --raid-devices 7 --level 6
>
> You will need a --backup-file to do this, on another device. Since you are
> keeping the same number of data discs before and after the reshape, the
> backup file will be needed throughout the reshape, so the reshape will take
> perhaps twice as long as a grow or shrink. If your backup-file is on the
> same disc(s) as md0 is (e.g. on another partition or array made up of other
> partitions on the same disc(s)), it will take way longer (gazillions of
> seeks), so I'd recommend a separate drive or if you have one a small SSD for
> the backup file.
>
> Doing the above with --layout=preserve will save you doing the reshape so
> you won't need the backup file, but there will still be an initial sync of
> the Q parity, and the layout will be RAID4-alike with all the Q parity on
> one drive so it's possible its performance will be RAID4-alike too i.e.
> small writes never faster than the parity drive. Having said that, streamed
> writes can still potentially go as fast as your 5 data discs, as per your
> RAID5. In practice, I'd be surprised if it was faster than about twice the
> speed of a single drive (the same as your current RAID5), and as Neil Brown
> notes in his reply, RAID6 doesn't currently have the read-modify-write
> optimisation for small writes so small write performance is liable to be
> even poorer than your RAID5 in either layout.
>
> You will never lose any redundancy in either of the above, but you won't
> gain RAID6 double redundancy until the reshape (or Q-drive sync with
> --layout=preserve) has completed - just the same as if you were replacing a
> dead drive in an existing RAID6.
>
> Hope the above helps!
>
> Cheers,
>
> John.
>
>

Hi,

Thanks for the replies. Allright, here we go;

 $ mdadm --grow /dev/md0 --bitmap=none
 $ mdadm --manage /dev/md0 --add /dev/sde1
 $ mdadm --grow /dev/md0 --verbose --layout=preserve  --raid-devices 7
--level 6 --backup-file=/root/md-raid5-to-raid6-backupfile.bin
mdadm: level of /dev/md0 changed to raid6

$ cat /proc/mdstat

                                                             Fri Apr
22 10:37:44 2011

Personalities : [raid6] [raid5] [raid4]
md0 : active raid6 sde1[7] sdg1[0] sdh1[6] sdf1[5] sdc1[3] sdd1[4] sdb1[1]
      9751756800 blocks super 1.2 level 6, 64k chunk, algorithm 18
[7/6] [UUUUUU_]
      [>....................]  reshape =  0.0% (224768/1950351360)
finish=8358.5min speed=3888K/sec

unused devices: <none>

And in dmesg:


 --- level:6 rd:7 wd:6
 disk 0, o:1, dev:sdg1
 disk 1, o:1, dev:sdb1
 disk 2, o:1, dev:sdd1
 disk 3, o:1, dev:sdc1
 disk 4, o:1, dev:sdf1
 disk 5, o:1, dev:sdh1
RAID conf printout:
 --- level:6 rd:7 wd:6
 disk 0, o:1, dev:sdg1
 disk 1, o:1, dev:sdb1
 disk 2, o:1, dev:sdd1
 disk 3, o:1, dev:sdc1
 disk 4, o:1, dev:sdf1
 disk 5, o:1, dev:sdh1
 disk 6, o:1, dev:sde1
md: reshape of RAID array md0
md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
md: using maximum available idle IO bandwidth (but not more than
200000 KB/sec) for reshape.
md: using 128k window, over a total of 1950351360 blocks.

IIRC there's a way to speed up the migration, by using a larger cache
value somewhere, no?

Thanks,
Mathias
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