Mathieu Benoit
667842b832
La doc et l'en-tête de l'algorithme affirmaient qu'au quatrième étage le noyau invité gelait « au même octet quelles que soient les ressources ». Lancer la descente l'a réfuté : son propre quatrième étage, à 2 vCPU, a démarré, s'est installé, et a écrit des gigaoctets. La VM examinée à la main en avait douze. Ce n'était donc pas un plafond d'imbrication mais un plafond de PARALLÉLISME sous imbrication — précisément ce que l'algorithme borne, et qui cesse ainsi d'être une supposition. Le « même octet », par ailleurs, ne voulait rien dire de ce qu'on lui faisait dire : 33 682 432 octets, c'est 32 Mio, la taille des fichiers d'amorçage. Retirer de la mémoire ne le déplaçait pas parce qu'il ne dépendait pas de la mémoire, pas parce qu'un mur absolu s'y trouvait. La conclusion — ne pas borner la RAM — reste juste ; sa justification était fausse. Une affirmation fausse dans la documentation est pire que pas de documentation : elle décide à la place du lecteur. Les deux passages disent maintenant ce qui a été mesuré, sur quoi, et ce que la descente a montré ensuite. --- EN --- The documentation and the algorithm's header claimed that at the fourth level the guest kernel froze "at the same byte whatever the resources". Running the descent refuted it: its own fourth level, at 2 vCPU, booted, installed, and wrote gigabytes. The VM examined by hand had twelve. So it was not a nesting ceiling but a PARALLELISM ceiling under nesting — exactly what the algorithm caps, which thereby stops being a guess. The "same byte", moreover, did not mean what it was made to mean: 33,682,432 bytes is 32 MiB, the size of the boot files. Removing memory did not move it because it did not depend on memory, not because an absolute wall sat there. The conclusion — do not cap RAM — still holds; its justification was wrong. A false claim in documentation is worse than no documentation: it decides in the reader's place. Both passages now say what was measured, on what, and what the descent showed afterwards. Assisted-by: Claude Opus 5 (cherry picked from commit b8c53eaf104f6891703e71b540c76ffd5994a2cb)
70 lines
3.3 KiB
Markdown
70 lines
3.3 KiB
Markdown
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# LongTest — tests that create real machines
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These are not unit tests. They create virtual machines, install systems on
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them, and take hours. They live here and **not** in `test/`, which the unit
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runner sweeps: `./script/test/run_unit_test.sh` must stay runnable in seconds
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on any machine, including one without virtualisation.
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Run them from the menu — `TODO › Execute › Test › Long tests` — or directly.
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## deep_proxmox.py — how deep does Proxmox-in-Proxmox go?
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The practicable nesting depth cannot be deduced, only measured — and one
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measurement is not a measurement.
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A manual look at one fourth-level VM found a guest **36 times slower than real
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time** (583 seconds of wall clock for 16 seconds of guest time, each ACPI line
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taking a second) and then a frozen kernel: identical RIP across three samples
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two minutes apart, and **not one byte written** to disk.
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Running this script **refuted the conclusion drawn from it**. Its own
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fourth-level VM — 2 vCPU where the manual one had 12 — booted, installed, and
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wrote gigabytes. What looked like a nesting ceiling was a *parallelism*
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ceiling under nesting. That is exactly what the algorithm caps, and this is
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how it stopped being a guess.
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Which is the point of the script: a number obtained once, on one machine, in
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one chain, is an anecdote.
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```
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./LongTest/deep_proxmox.py --depth 10 --dry-run # the plan, nothing created
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./LongTest/deep_proxmox.py --depth 10 # hours
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./LongTest/deep_proxmox.py --detruire # undo it
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```
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The descent is **uniform**. Every level, the first included, goes through the
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same six steps: create, wait for ssh, install Proxmox, reboot and check the
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kernel, bring pmxcfs back up, check the storage. Only creation differs —
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libvirt locally, `qm` afterwards.
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It sends **our** `install_proxmox.sh` over scp instead of letting the VM clone
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the repository: it is our code we want to exercise, and the remote is often
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behind the checkout — a fix absent from the remote made the same defect "come
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back" on three VMs in a row.
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### The resource algorithm
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Two things run out going down, and a third degrades. What runs out is
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arithmetic, and `script/proxmox/nesting.py` computes it:
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* **memory** — each level keeps what its own daemons need (`pve-cluster`,
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`pvestatd`, `pvedaemon`, `pveproxy`) before handing the rest down;
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* **disk** — the child's disk lives *inside* the parent's, which must also
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hold its own system.
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What degrades is measured, not assumed: past the second level, vendors
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document nothing. Hence one capped number — **2 vCPU** for every nested
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level. Twelve vCPU at the fourth level froze the guest kernel in early boot;
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the same two progressed. Bringing twelve processors online costs as many
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round trips through the whole stack.
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Memory is **not** capped. On that one manual VM, dropping it from 9 GB to
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2 GB moved nothing — it stopped after reading the same 32 MiB, which is simply
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the size of the boot files. Memory was not the lever; the vCPU count was. And
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trimming memory would starve the level below, which needs it to host the
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next.
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The plan is printed **before** anything is created, and the script never
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promises a depth it knows will not fit — better to announce six levels and
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reach six than to promise ten and die at the seventh without knowing why.
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