casync chunks the seed chunk store

[fhackenberger]

RAUC with casync seems to create a chunk index of the chunk index when installing a RAUC bundle.

root     29897  0.0  0.1 256164 10172 pts/0    Sl   18:36   0:00 rauc -d install /tmp/board-system-images-v3/update.casync.raucb
root     29922 85.9  0.1  48132 14444 ?        R    18:36  11:15 casync --log-level=debug --verbose extract --seed /run/rauc/rootfs.0 --store /tmp/board-system-images-v3/update.casync.castr --seed-output=no /run/rauc/bundle/rootfs.tar.gz.caidx /run/rauc/rootfs.1

ubuntu@sign-pc-18:~$ sudo du -xhcs /var/tmp/*.cased | sort -n     
3.3G    total
3.3G    /var/tmp/3a875a5021649923.cased

Logs from casync run by rauc:

Nov 12 18:48:54 sign-pc-18 rauc[29863]: Seeding var/tmp/3a875a5021649923.cased/ec82/ec828c3b0e9ca9277f2b36051edce47e8cae0e128e3fb21aeb70de8978c0ae99
Nov 12 18:48:54 sign-pc-18 rauc[29863]: Seeded var/tmp/3a875a5021649923.cased/ec82/ec828c3b0e9ca9277f2b36051edce47e8cae0e128e3fb21aeb70de8978c0ae99

In general it would be great, if casync could use reflinks when creating an uncompressed chunk store for a seed and rauc would set casync to use that. Not sure if this feature is available though. It would resolve a big issue with using casync (Less than 50% disk usage on slots is allowed).

[fhackenberger]

The solution for the bug is quite easy:
Write a .caexclude file with the content:
/var/tmp/*.cased
and store it in / before running casync. This should happen in casync_extract in src/update_handler.c if a seed is set. Maybe there is also a way to make that path explicit when calling casync, from what I understand casync creates a new random directory for the extract run, if --seed is specified.

[fhackenberger]

See #513 for a workaround, where a user created .caexclude file can be used to fix the initial casync make run.

[ejoerns]

Maybe the right option is to set TMPDIR to a device not being the seeded one? Should be combined with --exclude-submounts=yes if I get it right.

[fhackenberger]

In our current use case, with a large base image, having a separate tmpdir would create the need for a third large partition and would leave the issue with the very slow installation unresolved. So we went for the use-case I described in the PR. I'll close this now thanks for your feedback!

[ejoerns]

Maybe it is worth documenting the different RAUC/casync use cases together with their needs, pros and cons.
(And also write some hints on TMPDIR storage location).