Growing a raid array online with different capacity disks

Hello,

I would like to be able to take out a disk from a raid-5 array and add
a higher capacity disk, with the raid array growing in size.

a-la drobo/synology/qnap nas systems.

i.e. 4x1tb disks, take out 1x1tb replace with 1x3tb disk, array grows

Does mdadm support this?

I searched the official faq and debian faq for expand and grow and
didn't find anything.

Thanks a lot.

jbp
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Re: Growing a raid array online with different capacity disks

On Thu, 10 Mar 2011, jimbob palmer wrote:

> i.e. 4x1tb disks, take out 1x1tb replace with 1x3tb disk, array grows
>
> Does mdadm support this?

No.

> I searched the official faq and debian faq for expand and grow and
> didn't find anything.

How do you propose that a 4 drive RAID5 (giving you 3 tb usage) would
redundantly give you more usable space by swapping out one drive for a
larger one and still be fully redundant?

--
Mikael Abrahamsson email: swmike@swm.pp.se
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Re: Growing a raid array online with different capacity disks

2011/3/10 Mikael Abrahamsson :
> On Thu, 10 Mar 2011, jimbob palmer wrote:
>
>> i.e. 4x1tb disks, take out 1x1tb replace with 1x3tb disk, array grow=
s
>>
>> Does mdadm support this?
>
> No.
>
>> I searched the official faq and debian faq for expand and grow and
>> didn't find anything.
>
> How do you propose that a 4 drive RAID5 (giving you 3 tb usage) would
> redundantly give you more usable space by swapping out one drive for =
a
> larger one and still be fully redundant?

Oh I'm not proposing it, I'm asking if mdadm can do it, like the other
products can.

How do the others products manage it? x-raid I think one of them calls =
it.

>
> --
> Mikael Abrahamsson =A0 =A0email: swmike@swm.pp.se
>
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Re: Growing a raid array online with different capacity disks

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On Thu, 10 Mar 2011 14:24:24 +0100
jimbob palmer wrote:

> Oh I'm not proposing it, I'm asking if mdadm can do it, like the other
> products can.

No 'product' can 'do' a 1+1+1+3 TB RAID5, or it will not be a RAID5 but
something else.

--=20
With respect,
Roman

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Re: Growing a raid array online with different capacity disks

Yes the systems cited by the OP have nothing to do with RAID5, and in
fact are proprietary - definitely as in "non-standard", and I think
also none are open source.

They do seem to have a lot of flexibility - mix and match any number
and size of drives, toss another random-sized drive in an expand your
capacity. Apparently particularly well-suited to hobby/soho/sme users;
I've especially heard good things about Drobo for that market.

Think of the usual reaction a corporate IT guy (used to what he calls
"normal RAID", ie not mdadm) first hears about setting up mdadm RAID10
on odd numbers of drives, even say 3 - like "huh??!!"


On Thu, Mar 10, 2011 at 8:36 PM, Roman Mamedov wrote:
> On Thu, 10 Mar 2011 14:24:24 +0100
> jimbob palmer wrote:
>
>> Oh I'm not proposing it, I'm asking if mdadm can do it, like the other
>> products can.
>
> No 'product' can 'do' a 1+1+1+3 TB RAID5, or it will not be a RAID5 but
> something else.
>
> --
> With respect,
> Roman
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Re: Growing a raid array online with different capacity disks

> Yes the systems cited by the OP have nothing to do with RAID5, and in
> fact are proprietary - definitely as in "non-standard", and I think
> also none are open source.

QNAP systems are Linux based and the underlying RAID system is based
on mdadm. QNAP uses a newer version of mdadm that supports growing the
array through disk swapping as well as raid level migration.

I spent some time trying to understand how Drobo's "Beyond RAID"
system works and it's nothing special. All they do essentially is
partition the disks and make RAID arrays from within those partitions.
If you had two 500GB disks and two 1TB disks, Drobo would partition
the 1TB drives into 500GB chunks. then RAID the chunks. The first
500GB on all 4 drives would become a RAID-5 and the remaining
partitions on the 1TB drives would become a RAID-1.

Does Drobo use Linux under the hood? No idea. I've shied away from it
because the whole "BeyondRAID" felt kinda hokey.


--
Drew

"Nothing in life is to be feared. It is only to be understood."
--Marie Curie
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