When any MRtrix3 command must read or write image data, there are two primary mechanisms by which this may be performed:
Memory mapping: The operating system provides access to the contents of the file as though it were simply a block of data in memory, without needing to explicitly load all of the image data into RAM.
Preload / delayed write-back: When opening an existing image, the entire image contents are loaded into a block of RAM. If an image is modified, or a new image created, this occurs entirely within RAM, with the image contents written to disk storage only at completion of the command.
This design ensures that loading images for processing is as fast as possible and does not incur unnecessary RAM requirements, and writing files to disk is as efficient as possible as all data is written as a single contiguous block.
Memory mapping will be used by MRtrix3 wherever possible. However one circumstance where this should not be used is when write access is required for the target file, and it is stored on a network file system: in this case, the command typically slows to a crawl (e.g. progress bar stays at
0% indefinitely), as the memory-mapping implementation repeatedly performs small data writes and attempts to keep the entire image data synchronised.
MRtrix3 will now test the type of file system that a target image is stored on; and if it is a network-based system, it will not use memory-mapping for images that may be written to. However, if you
experience the aforementioned slowdown in such a circumstance, it is possible that the particular configuration you are using is not being correctly detected or identified. If you are unfortunate enough to encounter this issue, please report here the hardware configuration and file system type in use; hopefully MRtrix3 can then be updated to correctly handle that specific file system.