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2013 Mac Pro: SSD / Flash Drive Speed
Solid state drives (flash drives) that are SATA tend to top out around 540 MB/sec, and many deliver much lower write speed.
The OWC Mercury Accelsior PCIe SSD has been a stalwart performer for your author over 18 months in the 2010 Mac Pro. Two Accelsiors in a RAID-0 stripe on the 2010 Mac Pro deliver performance of around 1200 MB/sec.
Hence it astounds that the built-in Apple SSD in the 2013 Mac Pro achieves similar read speeds, though the Apple SSD write speeds are well below what a dual Accelsior RAID-0 stripe offers.
See the test machine configurations. Checked on two Mac Pro models with the 256GB SSD in both.
The performance of the built-in SSD is phenomenal. What a pity that one and only one such internal SSD is offered (only one slot).
The 1TB SSD performs substantially better than the 256GB SSD for write speed. However, the 1TB SSD was tested on OS X 10.9.2 (vs 10.9.1 for 256/512GB) and cannot be called definitive given the OS version change. Yet OWC tells me that this is the case, so it likey something that can be taken as a fact.
Few programs can use even 600 MB/sec effectively, so I/O as a performance issue goes away entirely for any sustained I/O activity.