Mount Namespace Isolation for Storage Management

[klogg]

Problem / Motivation

Currently, aos-unit processes directly access storage paths on the host filesystem, leading to several issues:

1. Tight coupling to host filesystem: Storage layout changes require script updates throughout codebase
2. Difficult overlay management: Integration with overlay storage (issue Support per-node writable overlay disks on top of a base image #1) and TPM state (issue Add optional TPM 2.0 emulation for QEMU nodes (swtpm + QEMU tpm-tis) #5) requires careful path coordination
3. Future multi-instance complexity: Supporting multiple instances (issue Support multiple aos-unit instances on one host #3) will require extensive path logic

Proposed Solution

Use Linux Mount Namespaces to provide a uniform filesystem view for aos-unit processes, decoupling scripts from the physical storage layout.

Key Benefits for Single Instance

1. Uniform paths: All scripts use same paths regardless of actual storage location
2. Path isolation: Processes cannot accidentally access directories outside their designated storage
3. Clean abstraction: Physical storage location hidden from application layer
4. Easier overlay integration: Overlays (issue Support per-node writable overlay disks on top of a base image #1) and TPM state (issue Add optional TPM 2.0 emulation for QEMU nodes (swtpm + QEMU tpm-tis) #5) use predictable paths
5. Future-proof: Enables multiple instance support (issue Support multiple aos-unit instances on one host #3) without script changes

Storage Layout

Host Filesystem

/var/lib/aos-unit/
├── aos-vm-main-amd64.qcow2          # Base images at root (backward compatible)
├── aos-vm-secondary-amd64.qcow2
├── aos-vm-custom-amd64.qcow2
│
└── overlays/
    └── default/                     # Default instance storage
        └── <all the stored data>

/var/log/aos-unit/
└── default/                          # Default instance logs
    └── <all the logs>

/run/aos-unit/
└── default/                          # Default instance runtime
    └── <all the runtime files>

Inside Mount Namespace

/var/lib/aos-unit/
├── aos-vm-main-amd64.qcow2          # Same as host (naturally visible)
├── aos-vm-secondary-amd64.qcow2
│
└── overlays/                         # Bind mounted from default/overlays/
    └── <all the stored data>

/var/log/aos-unit/                   # Bind mounted from default/
└── <all the logs>

/run/aos-unit/                       # Bind mounted from default/
└── <all the runtime files>

Acceptance Criteria

Functional Requirements

• Mount namespace created on aos-unit startup
• Bind mounts correctly map instance directories to uniform paths
• Base images accessible at /var/lib/aos-unit/*.qcow2 inside namespace
• Overlay storage works with mount namespace
• TPM state stored in correct location
• Logs written to correct location