Extendable raid - with soft-raid 5 and new mdadm?

Extendable raid - with soft-raid 5 and new mdadm?

[Stefan Neufeind]

Hi,

I was wondering how to set up an extendable raid, just for backup/data 
storage. It doesn't need to be high-performant - so imho I don't need a 
hardware-raid.

To provide a minimum of security I was wondering about using a soft-raid 
5 under Linux. However, I need to be able to extend the raid, if backup 
demand rises. Therefor I plan to use a raid 5 with three or four devices 
at the moment and need to be able to add more as the requirements grow.

I saw that in mdadm, version 1.7.0, there was a "grow"-feature 
introduced. However, at present it just works fine for raid 1 as far as 
I know. Can somebody tell me, if it will shortly (in the next few month) 
be possible to extend a soft-raid 5 with mdadm as well? What do you 
think about data-security? And do I need a plain raid 5 with ext3 on it 
or should I maybe put an LVM on top of raid 5 to be flexible?


Your feedback would be very much appreciated.

Regards,
  Stefan

Re: Extendable raid - with soft-raid 5 and new mdadm?

[Neil Brown]

On Sunday October 31, linux-raid@stefan-neufeind.de wrote:
> Hi,
> 
> I was wondering how to set up an extendable raid, just for backup/data 
> storage. It doesn't need to be high-performant - so imho I don't need a 
> hardware-raid.
> 
> To provide a minimum of security I was wondering about using a soft-raid 
> 5 under Linux. However, I need to be able to extend the raid, if backup 
> demand rises. Therefor I plan to use a raid 5 with three or four devices 
> at the moment and need to be able to add more as the requirements grow.
> 
> I saw that in mdadm, version 1.7.0, there was a "grow"-feature 
> introduced. However, at present it just works fine for raid 1 as far as 
> I know. Can somebody tell me, if it will shortly (in the next few month) 
> be possible to extend a soft-raid 5 with mdadm as well? What do you 
> think about data-security? And do I need a plain raid 5 with ext3 on it 
> or should I maybe put an LVM on top of raid 5 to be flexible?
> 

mdadm --grow
is unlikely the be able to add devices to a raid5 array in the near
future. 
There is a program called "raidreconfig" (I think) which can do this
off-line.   This is information only, not a recommendation as I have
never used it or looked at it.

NeilBrown

Re: Extendable raid - with soft-raid 5 and new mdadm?

[Doug Ledford]

On Mon, 2004-11-01 at 12:11 +1100, Neil Brown wrote:

> There is a program called "raidreconfig" (I think) which can do this
> off-line.   This is information only, not a recommendation as I have
> never used it or looked at it.

I've used it once.  It worked, it grew my raid5 array for me.  Took
forever, but it worked.

-- 
  Doug Ledford <dledford@redhat.com>     919-754-3700 x44233
         Red Hat, Inc.
         1801 Varsity Dr.
         Raleigh, NC 27606

Re: Extendable raid - with soft-raid 5 and new mdadm?

[Mike Hardy]

I also had success with raidreconfig and raid5, although I had a failure 
too. I was testing it to see if a strategy of adding disks as I needed 
them would work, and I discovered that it is *very* *very* touchy about 
the size of the partitions.

Where mdadm will just use the smallest partition as the limit for all 
units in the array, raidreconfig had an error related to partition sizes 
at the very end of its attempt to grow the array.

When I made the partition sizes the exact same block size it worked 
flawlessly, though slowly, in two separate tests, one going from 2x250GB 
to 3x250, and another going from 3x250 to 4x250.

I intend to use it to grow the array semi-indefintely, but I also have 
thorough, automatic backups so I may be more cavalier than people who 
care more about a specific instance of the data.

-Mike

Doug Ledford wrote:
> On Mon, 2004-11-01 at 12:11 +1100, Neil Brown wrote:
> 
> 
>>There is a program called "raidreconfig" (I think) which can do this
>>off-line.   This is information only, not a recommendation as I have
>>never used it or looked at it.
> 
> 
> I've used it once.  It worked, it grew my raid5 array for me.  Took
> forever, but it worked.
>