Snapshot 9.9s -> 0.13s. Restore 10.4s -> 0.80s. Three snapshots sharing
1.36 GB instead of costing 4.8 GB. The projected four-machine reset cycle
falls from ~90s, unbounded at worst, to ~15s dominated by a boot that
cannot be avoided. ADR 0029's inner-loop argument holds with copy-on-write
and did not without it.
The consistency matters as much as the speed: three consecutive snapshots
took 0.13, 0.12 and 0.13 seconds, against a dir second snapshot that never
finished.
One honest counter-observation recorded: launching onto the fresh
copy-on-write pool was slower, 20.2s against 14.3s, because the image had
to be unpacked into a pool that had never seen it. Paid once per pool, and
dwarfed by what snapshotting saves, but it went the other way.
Doing the measurement produced the answer to how the lab installs on a
clean machine, because both failure modes appeared while doing it.
Installed is not available: the daemon was present with units disabled and
no group. Issue 007.
Available is not adequate, and this is worse: with the storage tooling
absent everything worked and snapshots were seventy-six times slower.
Nothing failed, nothing warned. That is a variant the mesh has not
catalogued — its usual failure is reported success and did nothing; this is
reported success and did it seventy-six times slower, which no error
surface catches because nothing is wrong.
So the lab verifies CAPABILITY, never installation, and refuses to run
degraded rather than warning — a warning about a slow inner loop is read
once and ignored forever. Prerequisites may arrive from a mesh module or
from the lab's own bootstrap, and the second path is required rather than
convenient: a lab installable only by a mesh cannot host the development
of the mesh that installs it.
The lab is the second thing installed by hand, after the node host, and for
the same reason: something has to be first, and pretending otherwise
produces a circularity papered over by a script nobody exercises.
The lifecycle design closed on a question that was measurable rather than
arguable, so it was measured. One virtual machine on a workstation with
hardware virtualisation and NVMe.
Raising: the launch call returns in 3.4s, the machine is actually usable
after 14.3s. The gap is a design constraint — raise must wait for the
second number, because reporting the first would be transport reported as
effect, which is the mesh's own recurring failure.
Snapshot: 9.9s and 1.6 GB for a 1.5 GB instance. A dir snapshot is a full
copy; nothing is shared. Restore: 10.4s, usable again after 20.1s.
The second snapshot exceeded two minutes and never completed. That is the
more troubling number: snapshot cost here is not merely high, it is
unpredictable, and a loop with a variable multi-minute step is one nobody
trusts.
Projected to a four-machine scenario, a reset-and-rerun cycle is about a
minute and a half at best and unbounded at worst, before any of the mesh's
own work begins. That is too slow for an inner loop, and ADR 0029's whole
argument — that making the bootstrap path the inner loop turns the
least-exercised code into the most-exercised — holds only while resetting
is cheap.
The cause is not virtual machines. Hardware virtualisation is present and
machines boot in fourteen seconds. It is that the daemon offers exactly one
storage driver, dir, which has no copy-on-write and therefore no cheap
snapshot. The btrfs kernel module is available; btrfs-progs is simply not
installed, which is the entire reason the driver is absent.
The copy-on-write comparison was deliberately NOT run, because running it
would mean installing a package by hand — which the rules forbid and which
would have made the measurement unreproducible. So the honest statement is
that the current configuration is too slow and the likely fix is known but
unverified, rather than that btrfs fixes it.
jschoubben