Microsandbox /.msb Runtime Disk Quota

When a sandbox runtime path such as /.msb is host-backed and unbounded, guest writes can consume host disk even when other guest paths have quotas. The fix needs evidence from both sides: guest du/df, host bytes, deleted-open files, and a clear do-not-delete boundary for runtime state.

Runbook: Prove The Host-Backed Bytes

1. Identify the /.msb mount type and backing path before deleting anything. A runtime directory can contain overlays, rootfs state, sockets, metadata, or bind-mounted host storage.
2. Measure guest bytes with both du and df. A large gap usually means filesystem metadata, hidden mount contents, deleted-open files, reserved blocks, overlay lower/upper layers, or host-side sparse allocation.
3. Measure the host path at the same time. The important question is whether a guest write increases host disk usage one-for-one or through sparse/metadata amplification.
4. Check deleted-open files from inside the guest if tools are available. If a process still holds an unlinked file, du can drop while df stays high.
5. Add an explicit quota boundary for /.msb, not only /workspace, /tmp, or the OCI overlay upperdir.
6. Keep cleanup plan separate from quota policy. Quota prevents future host exhaustion; cleanup decides which stale runtime artifacts can be removed safely.

I would treat `/.msb` as a separate host-backed runtime filesystem, not as part of the normal guest quota surface.

Before adding cleanup, I would capture one read-only evidence block:

mount | grep ' /.msb '
findmnt -T /.msb -o TARGET,SOURCE,FSTYPE,OPTIONS
df -h /.msb
df -i /.msb
du -xhd1 /.msb 2>/dev/null | sort -h
lsof +L1 2>/dev/null | grep '/.msb' || true

Then compare that with the host-side backing directory at the same timestamps. If host usage increases with guest writes, `/.msb` needs its own quota/admission setting, for example `msb_size_mib`, because `/workspace`, `/tmp`, and overlay upper limits do not protect the host from this path.

For cleanup safety, I would not delete `/.msb` broadly. First separate runtime metadata, rootfs/overlay layers, sockets, active sandbox state, and stale scratch files. The acceptance test should prove:

- writes beyond the configured `/.msb` quota fail with ENOSPC
- `du`/`df` deltas are explained or surfaced in diagnostics
- deleted-open files are reported separately
- cleanup removes only stale owned artifacts, never active rootfs/runtime state

Do Not Delete First

• The whole /.msb tree, rootfs layers, active overlay upperdirs, sockets, or runtime metadata.
• Host backing directories until sandbox processes are stopped and open file handles have been checked.
• Any file used as proof of the du/df discrepancy before the bug has a regression test.
• Workspace data or guest state that belongs to an active sandbox session.