I tried to follow the official procedure to migrate from OfflienIMAP to SMD. I hit some difficulties, which I documented in upstream issues. What follows is the detailed test procedure I followed to test the synchronization and notes about the process.

  1. syncmaildir (SMD) configuration
  2. OfflineIMAP migration
  3. Full synchronization
  4. Performance comparison
  5. Open questions
  6. Issue summary

OfflineIMAP migration

This procedure was attempted to migrate my OfflineIMAP mailbox to SMD, but ultimately failed for reasons explained below.

  1. run smd-check-conf to create a template configuration in .smd/config.default and configure it with:

    SERVERNAME=smd-server-anarcat
    CLIENTNAME=curie-anarcat
    MAILBOX_LOCAL=Maildir-smd
    MAILBOX_REMOTE=Maildir-smd
    TRANSLATOR_RL="smd-translate -m oimap-dovecot -d RL default"
    TRANSLATOR_LR="smd-translate -m oimap-dovecot -d LR default"
    EXCLUDE="$MAILBOX_LOCAL/.notmuch/hooks/* $MAILBOX_LOCAL/.notmuch/xapian/*"
    
  2. authenticate remote server:

    ssh imap.anarc.at true
    
  3. created a .ssh/config entry

    # wrapper for smd
    Host smd-server-anarcat
        Hostname imap.anarc.at
        User anarcat
        BatchMode yes
        IdentitiesOnly yes
        Compression yes
    

    This will be useful later to configure the restricted shell account. A quick overview of those options:

    • Host: a unique alias that is unlikely to be reused outside of that configuration
    • Hostname: make sure we connect to the right host regardless of the alias defined in Host
    • User: same, but for the user
    • BatchMode: do not prompt so fails correctly if public key is missing
    • IdentitiesOnly: same, but do not look for external crypto tokens like a Yubikey
    • Compression: SMD does not do compression by default, so delegate that to SSH
  4. create the test maildirs, takes about 2 minutes, on both the client and the server:

    server$ cp -a Maildir/ Maildir-smd/
    client$ cp -a Maildir/Anarcat/ Maildir-smd/
    
  5. rename the INBOX folder on the client. the problem here is that the INBOX folder exists locally (thanks to offlineimap) and not remotely (thanks to dovecot). this was reported in bug #7 and it seems the workaround might be, on the client:

    client$ mv Maildir-smd/INBOX/{cur,new,tmp} Maildir-smd/ && rmdir Maildir-smd/INBOX/
    
  6. run smd-check-conf repeatedly until it stops complaining and looks sane. steps taken to cleanup remote directory:

    • removed top-level stray folders
    • moved out the Koumbit subdirectory that I couldn't make smd ignore. nothing seemed to work: not only did the folder not get ignored, the translation layer would fail to convert back and forth, because this was not really a Dovecot folder. i tried:

      EXCLUDE_REMOTE='Maildir/Koumbit Maildir/Koumbit* Maildir/Koumbit/* Maildir/Koumbit.INBOX.Archives.2012/ Maildir/.notmuch/hooks/* Maildir/.notmuch/xapian/*'

      filed as bug #4. The .notmuch folder ignores are necessary because smd crashes on symlinks (bug #5).

    • the above fails with remote folder as Maildir-smd. just removed the folders for now, but they are important and shouldn't be completely destroyed!

      server$ mkdir Maildir-smd-notmuch && mv Maildir-smd/.notmuch/{hooks,xapian,muchsync} Maildir-smd-notmuch

  7. when the config looks sane, the next step is to convert the folder away from OfflineIMAP idiosyncracies, particularly removing the X-OfflineIMAP header. there I have found that the suggested commandline:

    find Mail -type f -exec sed -i '/^X-OfflineIMAP/d' {} \;
    

    was problematic: it could corrupt email bodies as it works on complete messages, not just the headers. I wrote a generic header stripping script to workaround that issue. to call it, use this, which takes about three minutes, on both local and remote servers (because yes, the remote server also has OfflineIMAP headers somehow):

    server$ ~/dist/syncmaildir/misc/strip-header Maildir-smd/ 2>&1 >&2 | pv -s 187000 -l > log
    client$ ~/dist/syncmaildir/misc/strip-header Maildir-smd/ 2>&1 >&2 | pv -s 187000 -l > log
    

    contributed upstream as PR #3.

  8. then the files need to be renamed to please SMD, using, which takes about 3 minutes and generates ~140k renames (basically all files get renamed):

    client$ smd-uniform-names -v
    
  9. actually rename the files, using the script created, takes about 2 minutes:

    client$ sh -x smd-rename.sh 2>&1 | pv -l -s $(wc -l smd-rename.sh | cut -f1 -d ' ') > log-rename
    
  10. first dry-run pull, takes about 4 minutes. about 80k emails are missing, probably because SMD considers all folders, even if not subscribed (bug #2). all filenames to transfer are also dumped to stdout, ouch! (bug #6). at least logs are in ~/.smd/log/client.default.log

     client$ smd-pull --dry-run
    

    update: after a full rerun, it turns out only 9k emails are missing. after running the strip-header on both sides, only ~600. but with s/^cp/^mv/, back to 1400. grml. after another full rerun, the numbers are 401 new mails on pull.

    to get the count:

     grep stats::new-mails  /home/anarcat/.smd/log/client.default.log
    

    ... grouped per folder:

     grep stats::mail-transferre /home/anarcat/.smd/log/client.default.log | sed -e 's/ , /\n/g' | sed 's#/cur/.*##' |  sort | uniq -c | sort -n
    
  11. first dry-run push - problem: 15k mails missing from remote? maybe because we're in dry-run mode? need to backup remote and test again. nope. there are genuine diffs there, e.g. git-annex folder totally different. maybe not subscribed?

    client$ smd-push --dry-run
    

    result: 591 more duplicate emails.

  12. pull again: 490 mails deleted? push again, no change. wtf...

  13. create email remotely and locally, go to 10

  14. run smd-loop and hook into startup scripts? (TODO)

  15. create restricted shell (TODO)

  16. call notmuch new in ~/.smd/hooks/post-pull.d/ (TODO)

Clearing the slate can be done by running this command on both ends:

\rm -r .smd/workarea/ .smd/*.db.txt Maildir-smd/

The migration did not work very well: as documented above, lots of problems with exclude patterns, weird error messages, large dumps on the console, and scripts I had to rewrite. I also end up with duplicate emails in the process, something I generally try to avoid. Even if it's about 500 emails over ~200 000, it's still annoying.

I found it was better to start off with a clean slate and just copy all files as is to start with. The problem then of course is the directory layout is completely changed and is now incompatible with OfflineIMAP forever. But that is inevitable: the second we rename files and unmangle the headers to remove OfflineIMAP specific stuff, the folder cannot be reused, so it's unclear what the benefit of migrating from OfflineIMAP is over just using rsync to have a clean mirror to start with, avoiding all the messy rewrite rules logic.

Full synchronization

This is an alternative procedure from the above that just copies the files over and starts from a clean slate. Considering we lose compatibility with OfflineIMAP anyways, it seems like a much simpler procedure at no extra cost (except we need to copy files over).

The layout of the files will change to something a little more obscure which requires some fixes in notmuch tagging scripts and my notmuch-emacs config, but that will actually simplify things as we reduce the deltas between machines. It also requires a rescan of notmuch, which takes a long time (30-60 minutes), but that's a one-time cost only without data loss.

  1. configure SSH using the snippet from step 3 in the above procedure and confirm logging in works:

    ssh smd-server-anarcat
    
  2. create a copy on remote server, takes about two minutes:

    ssh anarc.at cp -a Maildir Maildir-smd
    
  3. strip offlineimap headers on remote, takes about 3-4 minutes?

    ssh anarc.at "~/dist/syncmaildir/misc/strip-header Maildir-smd/ 2>&1 >&2 | pv -s 187000 -l > log"
    
  4. synchronize the Maildir folder, this takes about 20 minutes: 1` ssh anarc.at "tar czf - Maildir-smd/" | pv -s 7G | tar xfz -

  5. cleanup IMAP server cruft copied from the remote folder:

    ( cd Maildir-smd
    find \( -name :list -o -name courierimapkeywords -o -name courierimapuiddb -o -name courierimapacl \) -a -delete
    find \( -name maildirfolder -o -name 'dovecot.index*' -o -name 'dovecot-uidlist' -o -name 'dovecot-keywords' \) -a -delete
    \rm dovecot-uidvalidity* dovecot.mailbox.log subscriptions )
    
  6. clear out notmuch metadata from remote and keep a dump of our current tags

    ( cd Maildir-smd
    \rm .notmuch/dump-201*
    \rm -r .notmuch/{muchsync,xapian,hooks}/ )
    cp -a Maildir/.notmuch/{xapian,hooks} Maildir-smd/.notmuch/
    notmuch dump > Maildir-smd/.notmuch/dump
    
  7. we go live. do one last offlienimap run, stop offlineimap, stop sending email locally, stop whatever writes to ~/Maildir/. on the server, this means stopping dovecot and postfix as well:

    killall -HUP offlineimap ; sleep 60 ; killall offlineimap
    ssh anarc.at sudo systemctl stop dovecot postfix
    ssh anarc.at mv Maildir Maildir.orig && mv Maildir-smd Maildir
    
  8. swap the old OfflineIMAP folder with the new folder.

    mv Maildir Maildir-offlineimap
    mv Maildir-smd Maildir
    

    also change the path to the OfflineIMAP folder in .offlineimaprc in case it gets started by mistake.

  9. change folders to point to real folders, change the translators, and fix the exclude patterns in ~/.smd/config.default:

    SERVERNAME=smd-server-anarcat
    CLIENTNAME=curie-anarcat
    MAILBOX=Maildir
    EXCLUDE="Maildir/.notmuch/hooks/* Maildir/.notmuch/xapian/*"
    TRANSLATOR_LR="smd-translate -m move -d LR default"
    TRANSLATOR_RL="smd-translate -m move -d RL default"
    
  10. run a test pull, should be no change:

    smd-pull --dry-run
    

    we had problems at this step previously because we were running smd on the Maildir-smd folder directly, before the rename. it might have been keeping state in ~/.smd/workarea that was confusing things so instead we made this direct procedure. the problem was reported upstream in bug #8, but it turns out the bug was my mistake, as I wasn't renaming the folder on the server side.

  11. test push, same:

    smd-push --dry-run
    
  12. run notmuch new. this involves first fixing the notmuch-tag and notmuch-purge script to remove the host-specific exception, then dropping all the unread, flagged and inbox tags that will be re-imported by mistake, then restoring from the backup:

    notmuch new
    notmuch tag -inbox -flagged -unread
    pv ~/Maildir/.notmuch/dump | notmuch restore
    

    running notmuch new the first time takes a loong time, maybe as long as if it was a clean database. might be worth not copying the xapian folder after all and just start from scratch, as long as we restore the previous dump.

  13. run the real pull/push

  14. send myself an email, which should create an email locally and remotely, run pull/push again

  15. hook notmuch new in SMD:

    echo '#!/usr/bin/notmuch new' > ~/.smd/hooks/post-pull.d/notmuch
    chmod +x ~/.smd/hooks/post-pull.d/notmuch
    
  16. start smd-loop from somewhere. for now just running in place of OfflineIMAP in a different workspace. I'm also experimenting with starting SMD from systemd

  17. create restricted shell. this means creating a new key with ssh-keygen, without password, and adding it on the server in ~/.ssh/authorized_keys with restrictions, like this:

    command="smd-restricted-shell",restrict ssh-ed25519 AAAAC3... user@host
    

    The restrict parameter assumes the server runs OpenSSH 7.2 or later, otherwise you need to spell out the individual restrictions like no-port-forwarding and so on, see authorized_keys(5) for details.

Performance comparison

OfflineIMAP is definitely slower. Here is a single run, although with a password input prompt that I estimate takes about one second:

*** Finished account 'Anarcat' in 0:13
11.46user 2.96system 0:13.39elapsed 107%CPU (0avgtext+0avgdata 84760maxresident)k
0inputs+112outputs (0major+51610minor)pagefaults 0swaps

That is with postsynchook disabled and running in "once" mode (-k Account_Anarcat:postsynchook=true -o), the hook (notmuch) takes about 3 seconds on its own.

SMD, on a snapshot of that mailbox about an hour old (so essentially the same):

$ time sh -c "smd-pull ; smd-push"
1.25user 0.82system 0:06.88elapsed 30%CPU (0avgtext+0avgdata 49180maxresident)k
0inputs+115240outputs (0major+89930minor)pagefaults 0swaps

So OfflineIMAP is at least two times slower, 8 full seconds overall. It definitely feels slower and clunkier. Even better, to fetch new mail we actually only need pull, which takes even less time:

$ time smd-pull
0.29user 0.04system 0:03.90elapsed 8%CPU (0avgtext+0avgdata 31480maxresident)k
0inputs+57592outputs (0major+5400minor)pagefaults 0swaps

Five times faster! And notice that much lower CPU usage.

Server-side usage is harder to diagnose, but I couldn't "see" smd on the server side during smd-pull at all: maybe it just popped in and out of existence without me noticing with top(1). But I definitely noticed Dovecot's imap process during the OfflineIMAP run. Circumstancial evidence from Prometheus monitoring shows a 18% CPU usage of Dovecot during fetches on marcos.

Running smd pull; smd push on a sleep 1 busy loop does make mddiff, lua5.1 and xdelta show up in top eventually, and CPU usage does seem higher than OfflineIMAP then (22%) - but it's a bit of an unfair comparison, because the updates are running much faster.

A fairer loop would be based on sleep 20 and would match better with the general OfflineIMAP loop frequency (-k Account_Anarcat:autorefresh=0.1 means 6 seconds plus the ~13 seconds run time). Such a loop converges to about 5% extra CPU usage.

Open questions

How should SMD be started in a session? User level systemd service? There is an "applet" that can be used, but that could be annoying. How else should errors be reported? It does look simple enough and non-intrusive: by default it doesn't notify for new mail, which is good.

It would be nice if notmuch insert could be used to deliver the emails locally instead of having to rescan the whole database.

How do we refresh a session? In OfflineIMAP, we used to SIGUSR, but it's unclear how that works in SMD. Maybe through the FIFOs?

Issue summary

This is a summary of the issues reported upstream, already mentioned above, in chronological order:

Trivia: at the time of writing, all of the currently reported issues against SMD are the ones above, reported by yours truly.

Created . Edited .