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For a server we have spec'd - HP Proliant DL370 G6 with a total of 3 SFF drive cages (up to 24 SFF drives total) - where would we use the SAS Expander vs. adding one or possibly two separate P410 Smart Array controllers?

Can each Drive Cage connect to a separate controller?

If yes, is cost the reason to use a SAS Expander vs. a separate P410 controller on each drive cage?

HP SAS Expander Card 468406-B21 (SFF) 2nd Drive Cage Kit 507803-B21

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SAS Expanders are most useful when you're going to connect a s***-ton of drives to a single server (think triple digits); as you might for an archive array. There's other reasons too, but less common in my experience. You're going to quickly hit bandwidth limitations when using them; so I would never recommend them if you can simply pop two RAID controllers in a server and have your needs satisfied.

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Having multiple controllers means that you can access the drives in parallel more efficiently. Using the SAS expander card means that you can create one logical volume using more than eight drives. If you don't need to make any logical volumes that span more than one drive cage, then there's not much advantage in the expander card.

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