I'm seeing that in order to mirror with Nexis, you need 3 engines.  Was this the case with ISIS?  I don't recall that requirment.  If this wasn't the case, what changed?  Thanks for any info!

The ISIS 7000/7500 offered Mirrored protection on a workspace. This would ensure that the file was evenly spread across the ISBs. If one ISB failed, it could be rebuilt from the remaining ISBs. This along with Network and System Director Redundancy provided no single point of failure and certainly when it was designed, it was very advanced.

The bottleneck for the rebuild was writing to the single ISB, it could only accept writes so quickly. If a second ISB failed before the first finished being rebuilt, it would cause data loss. On a large system with a lot of ISBs - it could easily have 192 ISBs - this was a very real risk. The entire stack had to participate in the rebuild, 191 ISBs were trying to write to one. This would cause a bandwidth hit during the entire time.

NEXIS protection is far superior to what was offered with ISIS. There is local Media Pack Protection, this is applied across the 10 Drives in each media pack and not across all the media packs, you can think of it as a local RAID but it's much better as it's media aware (like the 7000/7500 is) and only rebuilds the media on the drive, not the entire drive. This rebuild will only impact that media pack and not the entire stack.

Engine Protection is how Mirrored workspaces are created. An entire engine can fail and not only is there no data loss, there isn't any impact on workflow. A mirrored workspace can have 1 or 2 disk media pack protection. There's no requirement to have all the workspaces mirrored, the protection is based on the workspace, so one workspace can be mirrored with 2 disk and the next be unprotected.

Thanks Bob!  Do you know if that document is available online?