Re: LINEAR RAID, little help

[Rich <rich@pcfusion.co.uk>]

Gavin McCullagh wrote:
> 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?
>   
Er, I went with Linear as reading around people seemed to recommend this 
for odd sized drives (my old drives are 80's, 120 and 320's) also a read 
somewhere that data on the other drives is more recoverable that most of 
the other RAID's.
>   
>> 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 
>   
Well it contains stuff like TV and films a lot of which my friends have, 
so loosing it is not that big of a deal just inconvenient. As I said 
before I read linear was a better choice for odd sized drives and had 
better data recovery that the other RAID's
> "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
>
>   

I'm sure I read somewhere on the mdadm page in one of the posts about 
adding support for linear growing, and that was dated 2004 so it might 
be in there? Just wasn't to sure how to do it...


Thanks,

Rich :)
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