unison start the sync from zero when I change the parent folder

Question

I use unison to sync two folders that are in separate computers (via unison command with ssh). If for convenience I happen to change the name of the parent folder, unison start to sync the folders as if it were the first time I sync them, taking a lot of time (because the two folders are quite large). Here is the message I get:

Looking for changes
Warning: No archive files were found for these roots, whose canonical names are:
    /path/to/my/folder1
    /path/to/my/folder2
This can happen either
because this is the first time you have synchronized these roots, 
or because you have upgraded Unison to a new version with a different
archive format.  

Update detection may take a while on this run if the replicas are 
large.

Unison will assume that the 'last synchronized state' of both replicas
was completely empty.  This means that any files that are different
will be reported as conflicts, and any files that exist only on one
replica will be judged as new and propagated to the other replica.
If the two replicas are identical, then no changes will be reported.

Is there a way to tell unison that these folders have previously been synced? Perhaps changing the path somewhere and recovering the last unison file?

Similarly, maybe a related question: if I copy one of my folders to a new computer and therefore they are already in that moment identical, could I tell unison that they are identical to skip the update detection process?

Answer 1

You can (carefully) use the rootalias option. If you get this wrong you can completely break the mirror though so do take care.

The documentation (man unison) says,

rootalias xxx When calculating the name of the archive files for a given pair of roots, unison replaces any roots matching the left‐hand side of any rootalias rule by the corresponding right‐hand side.

What I think is not so well documented is the syntax of the rootalias parameter xxx. (It's defined in the section Path Specification but you have to know to look there and realise it applies to rootalias.)

Example for your situation

# Existing command (#1)
unison wasSrc remoteHost:dst

# Your attempted new command (#2)
unison newSrc remoteHost:dst

# Modified new command (#3)
unison -rootalias "//$(hostname)/$PWD/newSrc -> //$(hostname)/$PWD/wasSrc" newSrc remoteHost:dst

Once the modified new command (#3) completes successfully you can remove the -rootalias option and revert to the simpler version (#2).

Notes

• If wasSrc and newSrc are already absolute paths that start with / then you should not include $PWD but instead use this variant that omits $PWD:

   unison -rootalias "//$(hostname)/newSrc -> //$(hostname)/wasSrc" newSrc remoteHost:dst
  

• If you're not using bash or some other shell that maintains $PWD you can use $(/bin/pwd) or a hard-coded absolute path such as /home/user/path/to/newSrc