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Clarification of JBOD

Sometimes the definition of JBOD is given as "In JBOD multiple disk drives are attached along an array bus in an external cabinet. Once the array is attached to the host system I/O adapter, each drive in the array appeared as a separate volume to the operating system of the host system." And sometimes the definition given is "In some applications, however, it is desirable to be able to use all these disks as if they were one single volume. The proper term for this is spanning; the pseudo-cutesy term for it, clearly chosen to contrast against 'redundant array of inexpensive disks,' is Just A Bunch Of Disks or JBOD. How frightfully clever."

The above two definitions seem to be contradictory. It would be a great help if you could give some information on whether the individual disks in a JBOD system appear as a single volume or multiple addressable volumes to the host adapter? In either case, how will the devices appear to the host system?

Thanks in advance.


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JBOD disks appear as multiple discrete, addressable devices to the HBA. Higher-level volume management software can partition them, mirror them and apply whatever other techniques (RAID) to them. The point of JBOD is that there is no controller that receives I/Os from the host and transforms them into something that is done across multiple drives.

That said it is possible that a JBOD cabinet could be built where all the disk drives inside show up as LUNs behind a single Target ID. But this does not mean the drives are "spanned" in any way.

Regards,
Marc Farley

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This was first published in October 2001

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