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LINEAR RAID vs. LVM




On Mon, 25 Dec 2006, Tom Metro wrote:

> Daniel Feenberg wrote:
>> Notice that JBOD has the same rate of failure and storage capacity as RAID 
>> 0, but only half the data loss on each failure. We find that significant, 
>> but I never see it mentioned in the storage literature, or in newsgroups.
>
> I've always heard the definition of JBOD being quite literal - just a bunch 
> of disk. With nothing special about how they are logically arranged. But

That's what I understood. If I have 10 disks in a JBOD, I will need to 
mount at least 10 partitions, and none will depend on any other.

> apparently the definition is controversial, as indicated in this discussion:
>
> http://en.wikipedia.org/wiki/Talk:RAID#JBOD
>
> with a common second meaning being a concatenation of the physical disks into 
> one logical volume.

Well, there are two ways to use two disks without redundancy. You can 
interleve secotors or place one drive after the other. The former may gain 
you some speed over a single drive and has a name - RAID 0. I don't know 
the name of the other, or even if it is available. It would have the 
advantage that partitions might be expandable by adding drives. Either 
way, if I lose either drive I lose all the data.

>
> In Linux RAID terminology that seems to be referred to as Linear mode. It 
> seems focus has shifted to LVM for this functionality.
>
> That aside, I'd be curious to know which setup, if either, would prove to be 
> more easily (partially) recovered: a two drive Linear array or a two drive 
> LVM set, if one of the two drives fails.
>

I wouldn't think either would be recoverable - there isn't any redundancy 
and individual files could be spread across both drives. One can imagine a 
filesystem that covers multiple drives, and only loses some of the files 
when a drive dies, but I am not aware of one.


Daniel Feenberg

> -Tom
>
> -- 
> Tom Metro
> Venture Logic, Newton, MA, USA
> "Enterprise solutions through open source."
> Professional Profile: http://tmetro.venturelogic.com/
>


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