On Wed, Dec 30, 2009 at 09:05:53PM +0000, Chris G wrote:

My Western Digital NAS server offers me (among other RAID formats) both 'span' and 'JBOD'.

I thought they were synonyms for the same thing, that is using the disks 'as is' with no RAID processing. If they're different can anyone explain the difference please.

As always I find the answer myself soon after I post the question.

JBOD essentially means 'do nothing at all', the disks appear to the system as separate physical disks. Whereas 'span' means concatenate disks so that two (or more) physical disks appear to the system as one disk device.

The confusion is that, apparently, sometimes JBOD is used to mean the same as 'span' in that it can be interpreted as 'Just One Big Disk'. It's 'proper' meaning is 'Just a Bunch Of Disks'.

Have I got that about right, I'm quite happy for someone to tell me I have it totally wrong.