FileVault encryption slow when idle: opposite expected

Question

I have been encrypting a number of external disk using FileVault and have noticed puzzling behavior. The disks are external USB 3.0 disks.

When the process starts, I can see that the disk access proceeds at a reasonably snappy speed of ~35 MB/s using Activity Monitor.

If the computer is not touched for a while, the disk speed drops to a paltry ~5 MB/sec and will stay that way. As soon as I touch the mouse or press a key, it jumps back to the ~35 MB/sec speed and will stay that way until it goes idle again.

I would have expected just the opposite behavior. Keeping the disk accesses low while the user is active provides a snappy UI experience. Once the user is inactive, full bore access would seem to be the thing to do. Can anyone explain this behavior?

An example is shown here. Active disk while interacting with the computer, then slow disk after no interaction. As soon as the mouse was wiggled, the speed went back up.

Answer 1

There could be several reasons for this type of behaviour.

If you're on a battery-powered laptop, it might be that the system was designed to optimize run time on the battery. It could be that the 5 MB/sec mode is more power efficient than the 35 MB/sec mode.

Another reason could be that the encryption process needs good quality random numbers. It is often seen (across vendors, products and operating systems) that good quality random numbers require some form of user input or other external input to the system. Thus when the system is completely idle, the source of random numbers is slow (or runs out) - while just wiggling the mouse around can make the process much faster. It's not fully clear to me which part of the FileVault process you're working with, but I think you should be over the phase where random number generation is needed.