Is it possible to create a raid from/over other raids?

Is it possible to create a raid from/over other raids?

[Wilson Jonathan]

I realise if it is possible its not going to be the best solution, but a
temp stop gap... 

What I am thinking of doing is while I save up to by my 4'th new disk
for a new raid 6 setup (4 drive raid 6, currently has 3 disks 1 missing)
is it possible to use 3 existing smaller disks, create a raid0 across
them and then add the resulting /dev/dmX as "a disk" where the currently
missing disk is marked as missing.

EG. /dev/md7  ... /dev/sdb1 /dev/sdc1 /dev/sdd1 missing

mdadm create /dev/md8 raid0 /dev/sde1 /dev/sdf1 /dev/sdg1

mdadm --add /dev/md7 /dev/md8

If it is possible do I need to make sure the order in the conf file
first lists the /md8 then the /md7 or will mdadm see that a raid
contains a raid as a disk and then waits for it before assembling.

  

Re: Is it possible to create a raid from/over other raids?

[Adam Goryachev]

On 19/12/13 08:12, Wilson Jonathan wrote:
> I realise if it is possible its not going to be the best solution, but a
> temp stop gap...
>
> What I am thinking of doing is while I save up to by my 4'th new disk
> for a new raid 6 setup (4 drive raid 6, currently has 3 disks 1 missing)
> is it possible to use 3 existing smaller disks, create a raid0 across
> them and then add the resulting /dev/dmX as "a disk" where the currently
> missing disk is marked as missing.
>
> EG. /dev/md7  ... /dev/sdb1 /dev/sdc1 /dev/sdd1 missing
>
> mdadm create /dev/md8 raid0 /dev/sde1 /dev/sdf1 /dev/sdg1
>
> mdadm --add /dev/md7 /dev/md8
Yes, this is possible, MD doesn't care what the block device is.... MD 
uses block devices, and produces block devices...
> If it is possible do I need to make sure the order in the conf file
> first lists the /md8 then the /md7 or will mdadm see that a raid
> contains a raid as a disk and then waits for it before assembling.
This might depend on what distribution you use /etc... IMHO, it should 
work without a problem, but you should try it to confirm that.

IMHO, this isn't such a terrible idea, it adds at least some level of 
protection, although if you can using a 4th smaller disk with RAID5 
would be better still. I'm also assuming performance is not a critical 
factor in all this...

BTW, can anyone comment whether this would work:
Using linear, then only the portion of the RAID0 with the failed disk 
will be failed in the RAID6, so 2/3rds will still be 4 disk RAID6 and 
1/3rd will be 3 disk RAID6?

Regards,
Adam

-- 
Adam Goryachev Website Managers www.websitemanagers.com.au

Re: Is it possible to create a raid from/over other raids?

[Phil Turmel]

On 12/18/2013 05:37 PM, Adam Goryachev wrote:

[trim /]

> BTW, can anyone comment whether this would work:
> Using linear, then only the portion of the RAID0 with the failed disk
> will be failed in the RAID6, so 2/3rds will still be 4 disk RAID6 and
> 1/3rd will be 3 disk RAID6?

No, the entire linear array will be kicked out from under the raid6.  As
you said yourself, "MD uses block devices, and produces block devices".
 It doesn't look "under the hood" of an array's components.

Phil

Re: Is it possible to create a raid from/over other raids?

[Stan Hoeppner]

On 12/18/2013 3:12 PM, Wilson Jonathan wrote:
> I realise if it is possible its not going to be the best solution, but a
> temp stop gap... 
> 
> What I am thinking of doing is while I save up to by my 4'th new disk
> for a new raid 6 setup (4 drive raid 6, currently has 3 disks 1 missing)
> is it possible to use 3 existing smaller disks, create a raid0 across
> them and then add the resulting /dev/dmX as "a disk" where the currently
> missing disk is marked as missing.

The primary purpose of RAID is to protect your filesystem and files
against failure of a disk in an array.  The purpose of RAID6 is to
protect against a second disk failure or read error while you're
rebuilding the first failed drive in an array.  By creating a RAID6 with
one missing, you've essentially created a RAID5 array, as you can only
lose one drive.

Why didn't you simply create a RAID5, then reshape it to RAID6 when you
acquire your "4th" disk?  If you'd done that you wouldn't be worrying
about "plugging this hole".

> EG. /dev/md7  ... /dev/sdb1 /dev/sdc1 /dev/sdd1 missing
> 
> mdadm create /dev/md8 raid0 /dev/sde1 /dev/sdf1 /dev/sdg1
> 
> mdadm --add /dev/md7 /dev/md8
> 
> If it is possible do I need to make sure the order in the conf file
> first lists the /md8 then the /md7 or will mdadm see that a raid
> contains a raid as a disk and then waits for it before assembling.

It is possible, and yes the order matters.  md8 needs to be assembled
before md7 assembles.  But I don't think you really gain much by doing
this, especially when you're building the RAID6 constituent RAID0 member
out of old drives.

The tone of your request suggests the time frame to add the 4th drive is
now indeterminate, and you have a realization that it may possibly never
happen.  This being the case, I'd suggest you backup all your files,
blow away the 'kissing' RAID6, and make a fresh RAID5 of your 3 drives.
 Or a RAID10.  Then format and restore.  Your performance will increase,
and you won't have to worry about this missing drive in your crippled
RAID6 array.

-- 
Stan

Re: Is it possible to create a raid from/over other raids?

[Wilson Jonathan]

On Thu, 2013-12-19 at 03:17 -0600, Stan Hoeppner wrote:
> On 12/18/2013 3:12 PM, Wilson Jonathan wrote:
> > I realise if it is possible its not going to be the best solution, but a
> > temp stop gap... 
> 
> The primary purpose of RAID is to protect your filesystem and files
> against failure of a disk in an array.  The purpose of RAID6 is to
> protect against a second disk failure or read error while you're
> rebuilding the first failed drive in an array.  By creating a RAID6 with
> one missing, you've essentially created a RAID5 array, as you can only
> lose one drive.
> 
> Why didn't you simply create a RAID5, then reshape it to RAID6 when you
> acquire your "4th" disk?  If you'd done that you wouldn't be worrying
> about "plugging this hole".

I think I over thought things/was trying to be to clever :-/

> The tone of your request suggests the time frame to add the 4th drive is
> now indeterminate, and you have a realization that it may possibly never
> happen.  This being the case, I'd suggest you backup all your files,
> blow away the 'kissing' RAID6, and make a fresh RAID5 of your 3 drives.
>  Or a RAID10.  Then format and restore.  Your performance will increase,
> and you won't have to worry about this missing drive in your crippled
> RAID6 array.

The intention was to copy the existing raid6-6drive data (the end of the
new 3tb drives now holds/replaces the original 1tb drives) then copy the
data across to the new raid6 (held in the first 2tb of space on the
3-3tb disks) then destroy the 6 disk raid and finally re-use the disks
until a new drive can be purchased (was looking at 1-2 months time)
however thinking about it again and in light of your comments I think
I'll leave everything well alone and leave the 6 disk original running
(with 3 new drives)... when I've installed the missing 4th disk at that
point I will copy over the data, then tear down the 6 disk array, that
way I will have my 2 drive redundancy should any disk suffer an early
death. It will also mean that the existing 3 new disks will have had a
1-2 month workout, again allowing for 2 drive deaths in the original
array.

Re: Is it possible to create a raid from/over other raids?

[Stan Hoeppner]

md's ability to make arrays from partitions offers users great
flexibility, while simultaneously providing the novice enough rope to
tie his own noose...

You could have done worse.  I've read of cases of people making a RAID10
by taking two disks, laying two partitions on each, creating an md/RAID0
across the two partitions on one disk, then making a RAID1 across the
stripes.  50+ posts later, the members of that forum were finally able
to convince the guy why it wasn't working the way he envisioned...

On 12/19/2013 7:57 AM, Wilson Jonathan wrote:
> On Thu, 2013-12-19 at 03:17 -0600, Stan Hoeppner wrote:
>> On 12/18/2013 3:12 PM, Wilson Jonathan wrote:
>>> I realise if it is possible its not going to be the best solution, but a
>>> temp stop gap... 
>>
>> The primary purpose of RAID is to protect your filesystem and files
>> against failure of a disk in an array.  The purpose of RAID6 is to
>> protect against a second disk failure or read error while you're
>> rebuilding the first failed drive in an array.  By creating a RAID6 with
>> one missing, you've essentially created a RAID5 array, as you can only
>> lose one drive.
>>
>> Why didn't you simply create a RAID5, then reshape it to RAID6 when you
>> acquire your "4th" disk?  If you'd done that you wouldn't be worrying
>> about "plugging this hole".
> 
> I think I over thought things/was trying to be to clever :-/
> 
>> The tone of your request suggests the time frame to add the 4th drive is
>> now indeterminate, and you have a realization that it may possibly never
>> happen.  This being the case, I'd suggest you backup all your files,
>> blow away the 'kissing' RAID6, and make a fresh RAID5 of your 3 drives.
>>  Or a RAID10.  Then format and restore.  Your performance will increase,
>> and you won't have to worry about this missing drive in your crippled
>> RAID6 array.
> 
> The intention was to copy the existing raid6-6drive data (the end of the
> new 3tb drives now holds/replaces the original 1tb drives) then copy the
> data across to the new raid6 (held in the first 2tb of space on the
> 3-3tb disks) then destroy the 6 disk raid and finally re-use the disks
> until a new drive can be purchased (was looking at 1-2 months time)
> however thinking about it again and in light of your comments I think
> I'll leave everything well alone and leave the 6 disk original running
> (with 3 new drives)... when I've installed the missing 4th disk at that
> point I will copy over the data, then tear down the 6 disk array, that
> way I will have my 2 drive redundancy should any disk suffer an early
> death. It will also mean that the existing 3 new disks will have had a
> 1-2 month workout, again allowing for 2 drive deaths in the original
> array.
> 
> 
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