Performance with multiple instances of rsync across different direct-attached storage devices?

Question

Would running multiple instances of rsync with all the sources and destinations on different direct-attached storage devices cause any performance hits, or would the speed limits for each individual rsync transfer be due only to the read/write speeds of the direct-attached storage devices within the individual rsync commands themselves?

For instance, suppose I want to transfer:

Camera Card 1 to Direct-Attached Hard Drive 1

and

Camera Card 2 to Direct-Attached Hard Drive 2

If I run these rsync commands simultaneously, will the two rsync instances slow each other down, or will the speed limit for each individual instance still just be whatever the slowest device read/write speed is within each rsync instance?

Assume that the device interfaces are not bottlenecks.

Answer 1

I had an opportunity to run an experiment with this, so I figured I'd post an answer for anyone curious.

Any performance hit from the CPU is is imperceptible, if it even exists. As long as different devices aren't competing for the read or write speeds from other devices, there is no penalty to the rate of data transfer.

To illustrate, DAS A can transfer to DAS B at the same time DAS C transfers to DAS D, and the bottleneck for A to B is either A's read speed or B's write speed, unaffected by the other rsync instance transferring from C to D. Similarly, the rsync transfer from C to D is unaffected by the rsync instance transferring from A to B.