What is the fastest way to move a million images from one directory to another in Linux?

rsync would be a poor choice because it does a lot of client/server background work which accounts for local as well as remote systems.

mv is probably the best choice. If possible, you should try mv directory_old directory_new rather than mv directory_old/* directory_new/. This way, you move one thing instead of a million things.

find src_image_dir/ -type f -name '*.jpg' -print0 | xargs -0r mv -t dst_image_dir/ 

• This will not overflow argument expansion.
• You can specify the file extension, if you want to. (-name ...)
• find -print0 with xargs -0 allows you to use spaces in the names.
• xargs -r will not run mv unless there is something to be moved. (mv will complain if no source files are given).
• The syntax mv -t allows you to specify first the destination and then the source files, needed by xargs.
• Moving the whole directory is of course much faster, since it takes place in constant time regardless of the number of files contained in it, but:
  • the source directory will disappear for a fraction of time and it might create you problems;
  • if the process is using the current directory as output directory (in contrast to always referring to a full path from a non-moving location), you would have to relaunch it. (like you do with log rotation).

By the way, I would ask myself whether I really have to move such a big amount of files at once. Batch processing is overrated. I try not to accumulate huge amounts of work if I can process things at the moment they are generated.

If the two directories reside on the same filesystem, use mv on the DIRECTORY and not the contents of the directory.

If they reside on two different filesystems, use rsync:

rsync -av /source/directory/ /destination

Notice the trailing / on the source. This means it will copy the CONTENTS of the directory and not the directory itself. If you leave the / off, it will still copy the files but they will sit in a directory named /destination/directory. With the /, the files will just be in /destination

rsync will maintain file ownership if you run it as root or if the files are owned by you. It will also maintain the mtime of each individual file.