Linux – Sync directory across two servers using rsync (or alternative for syncing directories)

Create some sort of seed file deep in your non-tmpfs directory and only rsync back to non-tmpfs if it exists (meaning the "boot" copy worked), so something like:

BOOT

mount /path/tmpfs
rsync -aq --delete /path/non-tmpfs/ /path/tmpfs/

CRON

if [ -f /path/tmpfs/some/deep/location/filesgood.txt ]; then
  rsync -aq --delete /path/tmpfs/ /path/non-tmpfs/
fi

It's not perfect but, if you enhance that (by looking for 5 "cookie" files during cron if you want to in different directories, e.g.), it should be pretty safe.

rsync is a powerful tool, but it is, unfortunately, strangely picky about some of its pathnames.

If $DATA is an absolute path (i.e. it begins with a /), then the correct command-line to use is:

rsync -a --link-dest=$DATA $DATA/ $DATA/../upgrade_tmp

[Now, just a brief aside about rsync's strangeness. Note the trailing / added to the source argument. This tells rsync to work with the contents of the source directory, rather than with the source directory itself. (I'm assuming that $DATA doesn't already contain a trailing /.) In this case, we want to work with the contents, so we add the trailing /.]

If, on the other hand, $DATA is a relative path (i.e. it does not begin with a /), then Sean R's comment about --link-dest is bang on: The link-dest path is interpreted relative to the destination path, so you would use the following:

rsync -a --link-dest=../`basename $DATA` $DATA/ $DATA/../upgrade_tmp

EDIT

One final note, it turns out that the second rsync command-line I gave should work regardless of whether $DATA is an absolute path, since basename doesn't care whether a path is absolute or relative.