Re: mdadm - level change from raid 1 to raid 5

[NeilBrown <neilb@suse.de>]

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On Sun, 2 Oct 2011 16:24:48 +0200 Dominique <dcouot@hotmail.com> wrote:

> Hi Neil,
> 
> Thanks for the Info, I'll try a new series of VM tomorrow.
> 
> I do have a question though. I thought that RAID5 required 3 HDD not 2. 
> Hence I am a bit puzzled by your last comment....
> "Nope. This is because md won't change a 5-device RAID1 to RAID5. It 
> will only change a 2-device RAID1 to RAID5. This is trivial to do 
> because a 2-device RAID1 and a 2-device RAID5 have data in exactly the 
> same places. " Or do I grow to a 3HDD RAID5 config with a 'missing' HDD.

It is a common misunderstanding that RAID5 requires 3 drives, not 2.
2 is a perfectly good number of drives for RAID5.  On each stripe, on drive
holds the data, and the other drive holds the 'xor' of all the data blocks
with zero which results in exactly the data ( 0 xor D == D).
So a 2-drive RAID5 is nearly identical to a 2-drive RAID1, thus it is seen as
pointless and not considered to be a RAID5  (just as a triangle is not
considered to be a real quadrilateral, just because one of the 4 sides is of
length '0'!).
Some RAID5 implementations rule out 2-drive RAID5 for just this reason.
However 'md' is not so small-minded.
2-drive RAID5s are great for testing ... I used to have graphs showing
throughput for 2,3,4,5,6,7,8 drives - the '2' made a nice addition.
And 2-drive RAID5s are very useful for converting RAID1 to RAID5.  First
convert a 2-drive RAID1 to a 2-drive RAID5, then change the number of drives
in the RAID5.


RAID6 should really work with only 3 drives, but md is not so enlightened.
When hpa wrote the code he set the lower limit to 4 drives.  I would like to
make it 3, but I would have to check that 3 really does work and I haven't
done that yet.


> 
> I understand the 2HDD to 5HDD growth, but not how to make the other one. 
> Since I cant test it right know, I'll both tomorrow.

You really don't need too think to much - just do it.
You have a 2 drive RAID1.  You want to make a 5 drive RAID5, simply add 3
drives with
   mdadm /dev/md2 --add /dev/first /dev/second /dev/third

then ask mdadm to change it for you:
   mdadm --grow /dev/md2 --level=5 --raid-disks=5

and mdadm will do the right thing.
(Not that I want to discourage you from thinking, but sometimes experimenting
is about trying this that you don't think should work..)

NeilBrown

> 
> Dom
> 
> 
> On 01/10/2011 00:02, NeilBrown wrote:
> > On Fri, 30 Sep 2011 20:31:37 +0200 Dominique<dcouot@hotmail.com>  wrote:
> >
> >> Hi,
> >>
> >> Using Ubuntu 11.10 server , I am testing RAID level changes through
> >> MDADM. The objective is to migrate RAID 1 (1+ HDD) environment to RAID 5
> >> (3+ HDD) without data loss.
> >> In order to make as simple as possible, I started in a VM environment
> >> (Virtual Box).
> > Very sensible!!
> >
> >
> >> Initial Setup:
> >> U11.10 + 2 HDD (20GB) in Raid 1 ->  no problem
> >> The setup is made with 3 RAID 1 partition on each disk (swap (2GB), boot
> >> (500MB), and root (17,5GB)). I understand that this will allow to
> >> eventually grow to a RAID 5 configuration (in Ubuntu) and maintain boot
> >> on a RAID construct (swap and boot would remain on RAID 1, while root
> >> would migrate to RAID 5).
> >>
> >> Increment number of disks:
> >> add 3 HDD to the setup ->  no problem
> >> increase the RAID 1 from 2 HDD to 5 HDD ->  no problem, all disks added
> >> and synchronized
> > This is the bit you don't want.  Skip that step and it should work.
> >
> >
> >> root@ubuntu:~# cat /proc/mdstat
> >> Personalities : [linear] [multipath] [raid0] [raid1] [raid6] [raid5]
> >> [raid4] [raid10]
> >> md2 : active raid1 sda3[0] sde3[4] sdb3[1] sdc3[2] sdd3[3]
> >>         18528184 blocks super 1.2 [5/5] [UUUUU]
> >>
> >> md1 : active raid1 sda2[0] sde2[4] sdb2[1] sdd2[3] sdc2[2]
> >>         488436 blocks super 1.2 [5/5] [UUUUU]
> >>
> >> md0 : active raid1 sdb1[1] sde1[4] sda1[0] sdc1[2] sdd1[3]
> >>         1950708 blocks super 1.2 [5/5] [UUUUU]
> >>
> >>
> >> Change Level:
> >> That's where the problem occurs:
> >> I initially tried 3 different approaches for md2 (the root partition)
> >>
> >>       1. Normal boot
> >>
> >>       mdadm /dev/md2 --grow --level=5
> >>
> >>       Not working: 'Could not set level to raid 5'. I suppose this is
> >> because the partition is in use. Makes sense.
> > Nope.  This is because md won't change a 5-device RAID1 to RAID5.  It will
> > only change a 2-device RAID1 to RAID5.  This is trivial to do because a
> > 2-device RAID1 and a 2-device RAID5 have data in exactly the same places.
> > Then you can change your 2-device RAID5 to a 5-device RAID5 - which takes a
> > while but this can all be done while the partition is in use.
> >
> > i.e. if you start with a RAID1 with 2 active devices and 3 spares and issue
> > the command
> >      mdadm /dev/md2 --grow --level=5 --raid-disks=5
> >
> > it will convert to RAID5 and then start reshaping out to include all 5 disks.
> >
> >
> > NeilBrown


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