Re: LINEAR RAID, little help

[Gavin McCullagh <gmccullagh@gmail.com>]

Hi,

The main reason I'm posting (given others can answer these questions
better) is to ask a further question: 

Why would anyone use RAID-linear?  If RAID-0 gives better performance for
the same (reduced) reliability, what's the point of using Linear?  Do you
get slightly more space out of it?  I presume it's for stuff where you
don't place much value no your data.

On Sat, 07 Apr 2007, Rich wrote:

> I currently have a linear RAID setup via mdadm with is made up of 3 
> drives. I just have a few questions that I can't seem to find searching 
> around on Google, etc.

I must admit I've never used linear raid.  May I ask what made you choose
it over say raid-0?

> First question, what happens if one drive fails (I know I will loose the 
> data on that drive) but how, if at all can I recover the data on the 
> other drives can I plug them in (on their own) as though they were in 
> fact an individual drives? Do I need to execute a rebuild command in 
> mdadm at all to rebuild the array?

http://tldp.org/LDP/EVMSUG/html/characraidlvls.html

I'm sure someone else will give you a better answer but it would appear
from these docs that your situation is similar to that of a person using
RAID-0 who has a drive failure.  Basically, you end up with a big portion
of your filesystem data missing so you may lose everything.  I guess you
might be able to recover some data using some filesystem recovery tools but
it's likely to be a pretty unreliable process and likely to vary from one
filesystem to the next.  It's probably a little more reliable than RAID-0
in that what's left is probably contiguous but I would certainly not want
to rely on such a process for my data.

The faq says similar "RAID-linear is a simple concatenation of partitions
to create a larger virtual partition. It is handy if you have a number
small drives, and wish to create a single, large partition. This
concatenation offers no redundancy, and in fact decreases the overall
reliability: if any one disk fails, the combined partition will fail."

http://tldp.org/HOWTO/Software-RAID-0.4x-HOWTO-2.html

> Second question, how can I go about adding a drive to my linear RAID, I
> wish to add two new 500GB drives but I'm unsure how. I have found howto's
> for RAID 5 but I just wanted to check it was a similar process for
> Linear? Also this won't effect any data currently on the drive will it?

You don't say what you're doing on this array, but before modifying it, I'd
be seriously inclined to question whether RAID-linear is really the right
thing to be using at all.  Anyway, the mdadm manpage says 

"Currently the only support available is to
 · change the "size" attribute for RAID1, RAID5 and RAID6.
 · increase the "raid-disks" attribute of RAID1 and RAID5.
 · add a write-intent bitmap to any array which support these bitmaps, or
   remove  a  write-intent bitmap from such an array."

which suggests you can't.  That might be wrong though as it sounds (to me
anyway) like linear would be one of the easier ones to implement grow for.
I'm not sure.

Gavin

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