Each JBOD includes two expanders. One expander in each JBOD is connected to each storage server for redundancy. The Petarack employs SAS drives, which feature two ports, facilitating a connection to each server, again, for redundancy. The idea is that if one server or one HBA drops offline, there is always another data path available.
Vibration is a big factor in a chassis with so many disks. If you were to build your own enclosure with 360 drives and not use any sort of rotational vibration correction mechanism, performance would degrade by about 50% (according to Aberdeen), and you would see multiple drive failures in a short period of time. More specifically, the vibration would cause read errors, and the hard drives would repeatedly try to reread and correct them. Not only does this lower the performance, but it also wears the disks out, reducing lifespan.
For that reason, 3 TB nearline SAS drives make the most sense in an environment like this. They offer the reliability characteristics of an enterprise product with the capacity advantages of magnetic SAS-based storage. Aberdeen says the Petarack also supports 15 000 RPM SAS drives or SSDs for performance-sensitive applications. Multiple technologies can even coexist within a single Petarack, though obviously that’ll affect its ability to host an actual petabyte of storage space.