On Mon, 15 Apr 2019 15:45:56 +0200
Andreas Klauer <Andreas.Klauer@xxxxxxxxxxxxxx> wrote:
> On Mon, Apr 15, 2019 at 02:04:09PM +0500, Roman Mamedov wrote:
> > I have a 2-device RAID0 with mdadm. I would like to replace one disk with
> > a new one, while keeping the array online.
>
> Well, it's possible ... just not intuitive. Maybe a bit risky as well?
>
> | set up some test devices with random data
>
> # truncate -s 100M a b c
> # shred -v -n 1 a b c
> # losetup --find --show a
> /dev/loop0
> # losetup --find --show b
> /dev/loop1
> # losetup --find --show c
> /dev/loop2
>
> | create a 2-device RAID 0 and checksum
>
> # mdadm --create /dev/md42 --level=0 --raid-devices=2 /dev/loop[01]
> # md5sum /dev/md42
> 7d1bba7a97467fd97d60d5e0303e4785 /dev/md42
>
> | replace /dev/loop0 with /dev/loop2
>
> # mdadm --grow /dev/md42 --level=5
> mdadm: level of /dev/md42 changed to raid5
At this point it starts rewriting data on both member devices into RAID5:
[93729.643620] md/raid:md0: device sdg3 operational as raid disk 1
[93729.643622] md/raid:md0: device sdi3 operational as raid disk 0
[93729.674327] md/raid:md0: raid level 5 active with 2 out of 3 devices, algorithm 5
[93729.750306] md: reshape of RAID array md0
md0 : active raid5 sdg3[1] sdi3[0]
726982656 blocks super 1.2 level 5, 1024k chunk, algorithm 5 [3/2] [UU_]
[>....................] reshape = 1.5% (5624832/363491328)
finish=51.1min speed=116584K/sec
Not what I was expecting. And surely converting back to RAID0 require another
rewrite, i.e. this process is not a good solution either.
> # mdadm --manage /dev/md42 --add-spare /dev/loop2 --replace /dev/loop0 --with /dev/loop2
> mdadm: added /dev/loop2
> mdadm: Marked /dev/loop0 (device 0 in /dev/md42) for replacement
> mdadm: Marked /dev/loop2 in /dev/md42 as replacement for device 0
> # mdadm --wait /dev/md42
> # mdadm --grow /dev/md42 --level=0
> mdadm: level of /dev/md42 changed to raid0
>
> | see if data is still intact
>
> # md5sum /dev/md42
> 7d1bba7a97467fd97d60d5e0303e4785 /dev/md42
>
> Regards
> Andreas Klauer
--
With respect,
Roman
