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Promise Pegasus J4 — SSD Speed (Compressible)
Tested on the 2.7 GHz MacBook Pro with Retina display, which has a Thunderbolt connector. This test is for compressible data.
Update: see issues.
Sustained transfer speed — compressible data
The Thunderbolt v1 bus speed effectively throttles peak performance to just under 900 MB/sec for reads. Observe that 2/3/4 drives striped all effectively perform the same for reads of compressible data.
Since read activity dominates most workflow scenarios, it is fair to say that even two OWC 6G SSDs in this Pegasus J4 will provide enviably robust performance.
Write speeds are lower, and this appears to be a Thunderbolt or Pegasus limitation, because write speeds on a native SATA 6Gbps connector approach 500MB/sec for a single SSD.
But none of that really matters: the speeds with even two SSDs are well beyond the ability of nearly every program to use the high speed in any meaningful way. Even Photoshop CS6 sees greatly diminished returns after ~600MB/sec.
For compressible data, the sweet spot for reads and writes is the triple-SSD configuration, which leaves that 4th bay free for a high capacity hard-drive utilized as a clone backup.