RAID10 reshape and change lout possible?

RAID10 reshape and change lout possible?

[Reindl Harald]

since in recent news i saw that convert several raid-levels with respahe 
is possible now

is it somehow possible to change a existing md RAID10 array from "2 
near-copies" to "2 far-copies" and what would be the mdadm syntax? the 
IO intense workload here are large reads within virtual disks and as far 
as i can see far-layout would here be the better option but as installed 
the stuff in 2011 not sure if anaconda even offered that in the OS installer

so i would like to change the layout "on-the-fly" without dump/restore 
data (which is hard for the OS itself while keep dracut and friends 
working just fine) and also important keep all the UUID's (there is a 
second cloned machine which is in large parts rsynced including 
grub-config and fstab)

Re: RAID10 reshape and change lout possible?

[Andy Smith]

Hello,

On Sun, Feb 26, 2017 at 03:37:24PM +0100, Reindl Harald wrote:
> is it somehow possible to change a existing md RAID10 array from "2
> near-copies" to "2 far-copies" and what would be the mdadm syntax?

https://manpages.debian.org/jessie/mdadm/mdadm.8.en.html suggests
that --grow --layout can be changed on RAID-10. I have never tried
it myself. Experiment with loop devices. It looks like the syntax
would be:

# mdadm --grow /dev/mdX --layout=f2

Cheers,
Andy

-- 
https://bitfolk.com/ -- No-nonsense VPS hosting

> The optimum programming team size is 1.
Has Jurassic Park taught us nothing?         — pfilandr

Re: RAID10 reshape and change lout possible?

[Reindl Harald]

Am 26.02.2017 um 19:51 schrieb Andy Smith:
> On Sun, Feb 26, 2017 at 03:37:24PM +0100, Reindl Harald wrote:
>> is it somehow possible to change a existing md RAID10 array from "2
>> near-copies" to "2 far-copies" and what would be the mdadm syntax?
>
> https://manpages.debian.org/jessie/mdadm/mdadm.8.en.html suggests
> that --grow --layout can be changed on RAID-10. I have never tried
> it myself. Experiment with loop devices. It looks like the syntax
> would be:
>
> # mdadm --grow /dev/mdX --layout=f2

thanks for your answer

the syntax seems to be correct given the error message says clear 
"Cannot reshape RAID10 to far-mode" which sadly answers the question - 
at least for now, maybe that will change in the future

[root@testserver:~]$ mdadm --grow /dev/md0 --layout=f2
mdadm: Cannot reshape RAID10 to far-mode

[root@testserver:~]$ rpm -q mdadm
mdadm-3.4-3.fc24.x86_64

[root@testserver:~]$ uname -a
Linux testserver.rhsoft.net 4.9.12-100.fc24.x86_64 #1 SMP Thu Feb 23 
21:29:13 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

Re: RAID10 reshape and change lout possible?

[Andy Smith]

Hi,

On Sun, Feb 26, 2017 at 09:28:34PM +0100, Reindl Harald wrote:
> Am 26.02.2017 um 19:51 schrieb Andy Smith:
> >https://manpages.debian.org/jessie/mdadm/mdadm.8.en.html suggests
> >that --grow --layout can be changed on RAID-10.

[…]

> [root@testserver:~]$ mdadm --grow /dev/md0 --layout=f2
> mdadm: Cannot reshape RAID10 to far-mode

Hmm, that man page says that Debian jessie has version 3.3.2-5 and
still it says:

Grow

[…]

    …changing the chunk size and layout for RAID 0,4,5,6,10 as well
    as adding or removing a write-intent bitmap.

So is the man page wrong?

Admittedly until I looked I thought RAID-10 couldn't be reshaped at
all in any way. And now I don't know at all. :)

Cheers,
Andy

-- 
https://bitfolk.com/ -- No-nonsense VPS hosting

Re: RAID10 reshape and change lout possible?

[Reindl Harald]

Am 26.02.2017 um 22:17 schrieb Andy Smith:
> Hi,
>
> On Sun, Feb 26, 2017 at 09:28:34PM +0100, Reindl Harald wrote:
>> Am 26.02.2017 um 19:51 schrieb Andy Smith:
>>> https://manpages.debian.org/jessie/mdadm/mdadm.8.en.html suggests
>>> that --grow --layout can be changed on RAID-10.
>
> […]
>
>> [root@testserver:~]$ mdadm --grow /dev/md0 --layout=f2
>> mdadm: Cannot reshape RAID10 to far-mode
>
> Hmm, that man page says that Debian jessie has version 3.3.2-5 and
> still it says:
>     …changing the chunk size and layout for RAID 0,4,5,6,10 as well
>     as adding or removing a write-intent bitmap.
>
> So is the man page wrong?
>
> Admittedly until I looked I thought RAID-10 couldn't be reshaped at
> all in any way. And now I don't know at all. :)

well, change to "offset" seems to be supported (also not that long at 
all) if there is enough room and Google round confirms that

[root@testserver:~]$ mdadm --grow /dev/md0 --layout=o2
mdadm: insufficient head-room on /dev/sdg1

with the knowledge from today i would have chosen RAID1+0 or two RAID1 
and LVM stripe on top at least for the data partitions which would 
support the current 4 disk setup with "writemostly" and so achieve the 
full read performance out of the two SSD instead need another 2 SSD 
disks or build the raid-setup from scratch, both not making me terrible 
happy :-)

Re: RAID10 reshape and change lout possible?

[Brad Campbell]

On 26/02/17 22:37, Reindl Harald wrote:
> since in recent news i saw that convert several raid-levels with respahe
> is possible now
>
> is it somehow possible to change a existing md RAID10 array from "2
> near-copies" to "2 far-copies" and what would be the mdadm syntax? the
> IO intense workload here are large reads within virtual disks and as far
> as i can see far-layout would here be the better option but as installed
> the stuff in 2011 not sure if anaconda even offered that in the OS
> installer

Far layout will significantly improve read performance, however it will 
also significantly impact write performance. You really want to spool up 
a test system and benchmark the performance in your specific application 
before considering changes like this. The write performance degradation 
on magnetic media is significant due to the extra seeking required.

As you've already discovered. It's a backup and restore exercise on RAID10.

Regards,
Brad