mos_volmgr

MOS Volume Management — a Rust workspace for storage management on MOS (Mycelium Operating System) nodes.

Architecture

                 ┌─────────────────────────────────────┐
                 │          mos_volmgr_common           │
                 │   shared storage primitives (lib)    │
                 │  device discovery, GPT partitioning, │
                 │  mkfs, mount, RAID, subvolumes       │
                 └──────────┬──────────┬───────────────┘
                            │          │
               ┌────────────┘          └────────────┐
               │                                    │
    ┌──────────▼──────────┐          ┌──────────────▼──────────────┐
    │     mos_sysvol      │          │        mos_volmgrd          │
    │  boot-time oneshot  │          │  volume management daemon   │
    │  (binary: mos_sysvol)│          │  JSON-RPC over Unix socket │
    └─────────────────────┘          │  (binary: mos_volmgrd)     │
                                     └──────────────▲──────────────┘
                                                    │ Unix socket
                                     ┌──────────────┴──────────────┐
                                     │        mos_volmgr           │
                                     │    CLI client for daemon    │
                                     │  (binary: mos-volmgr)      │
                                     └─────────────────────────────┘

Crate	Binary	Role
mos_volmgr_common	(library)	Shared primitives: sysfs device discovery, GPT via gptman, mkfs wrappers, mount/unmount, mdadm RAID, btrfs subvolumes, constants
mos_sysvol	mos_sysvol	Boot-time oneshot run by my_init. Detects, partitions, formats, and mounts persistent storage. Fixed 5-partition layout, RAID1 dual-disk support
mos_volmgrd	mos_volmgrd	Long-running daemon. Listens on /run/mos_volmgrd/mos_volmgrd.sock for JSON-RPC 2.0 requests. Queries, manages, and monitors storage
mos_volmgr	mos-volmgr	CLI client. Connects to the daemon, sends RPC calls, prints results

Building

cargo build --release

Binaries are produced at:

• target/release/mos_sysvol
• target/release/mos_volmgrd
• target/release/mos-volmgr

Boot Flow

1. mos_sysvol runs at boot (called by my_init as a oneshot service)

   • Loads btrfs kernel module
   • Checks for existing MOS storage by label
   • If found: assembles RAID arrays, mounts everything
   • If not found: discovers empty disks, partitions (GPT), formats, creates subvolumes, mounts
   • Configuration via kernel cmdline: mossize=N (data GB, default 4), mosswap=N (swap GB, default 2)

2. mos_volmgrd starts after mos_sysvol completes

   • Binds Unix socket at /run/mos_volmgrd/mos_volmgrd.sock
   • Accepts JSON-RPC 2.0 connections
   • Manages storage for the lifetime of the node

3. mos-volmgr is used interactively or from scripts to query and manage storage

CLI Usage

Querying Storage

# Full inventory (disks, partitions, filesystems, mounts, subvolumes)
mos-volmgr status

# Individual queries
mos-volmgr disks
mos-volmgr partitions
mos-volmgr filesystems
mos-volmgr mounts
mos-volmgr subvolumes

# Free space on a specific disk
mos-volmgr free-space /dev/sda

# Which storage topologies are possible with current disks
mos-volmgr topologies

Statistics and Health

# Filesystem usage (size, used, available, percent)
mos-volmgr usage

# Btrfs device error counters
mos-volmgr device-stats /var/cache/system

# Btrfs scrub status
mos-volmgr scrub-status /var/cache/system

# Detailed btrfs filesystem usage breakdown
mos-volmgr fs-usage /var/cache/system

# Btrfs data/metadata/system allocation
mos-volmgr fs-df /var/cache/system

Managing Volumes

Creating a Partition

Create a new partition in unallocated space on an existing GPT disk:

# Use all remaining free space
mos-volmgr create-partition \
  --disk /dev/sda \
  --name mydata \
  --part-type linux

# Specify a size (in bytes)
mos-volmgr create-partition \
  --disk /dev/sda \
  --name extra \
  --part-type linux \
  --size 107374182400  # 100 GiB

Partition types: linux, esp, swap, bios_boot.

Creating a Filesystem

# btrfs
mos-volmgr create-filesystem \
  --device /dev/sda6 \
  --fstype btrfs \
  --label MYDATA

# ext4
mos-volmgr create-filesystem \
  --device /dev/sda6 \
  --fstype ext4 \
  --label MYEXT4

# Also: vfat, swap, bcachefs

Mounting and Unmounting

mos-volmgr mount \
  --source /dev/sda6 \
  --target /mnt/mydata \
  --fstype btrfs \
  --options "noatime,compress=zstd"

mos-volmgr unmount /mnt/mydata

Subvolume Management

# Create a new subvolume on the MOSDATA filesystem
mos-volmgr create-subvolume myapp-data

# Delete a subvolume (refuses to delete core subvolumes: system, etc, modules, vm-meta)
mos-volmgr delete-subvolume myapp-data

# Create a read-only snapshot
mos-volmgr snapshot system system-snap-20260325

# Set a quota (bytes)
mos-volmgr set-quota myapp-data 10737418240  # 10 GiB

Filesystem Utilities

# Start a scrub (background integrity check)
mos-volmgr scrub-start /var/cache/system

# Check scrub progress
mos-volmgr scrub-status /var/cache/system

# Start a balance (rebalance data across devices)
mos-volmgr balance-start /var/cache/system

# Check balance progress
mos-volmgr balance-status /var/cache/system

# Defragment a path recursively
mos-volmgr defrag /var/cache/system

# Grow filesystem to fill available device space
mos-volmgr resize-max /var/cache/system

JSON Output

All commands accept --json for machine-parseable output:

mos-volmgr disks --json
mos-volmgr usage --json
mos-volmgr create-subvolume mydata --json

Service Discovery

# Show the OpenRPC service document (all available methods)
mos-volmgr discover

JSON-RPC API

The daemon speaks JSON-RPC 2.0 over a Unix socket at /run/mos_volmgrd/mos_volmgrd.sock. Each request is a single-line JSON object terminated by \n.

Direct access with socat

# List disks
echo '{"jsonrpc":"2.0","id":1,"method":"storage.list_disks","params":{}}' | \
  socat - UNIX-CONNECT:/run/mos_volmgrd/mos_volmgrd.sock

# List free space on a disk
echo '{"jsonrpc":"2.0","id":1,"method":"storage.list_free_space","params":{"disk":"/dev/sda"}}' | \
  socat - UNIX-CONNECT:/run/mos_volmgrd/mos_volmgrd.sock

# Create a subvolume
echo '{"jsonrpc":"2.0","id":1,"method":"storage.create_subvolume","params":{"name":"mydata"}}' | \
  socat - UNIX-CONNECT:/run/mos_volmgrd/mos_volmgrd.sock

# OpenRPC service discovery
echo '{"jsonrpc":"2.0","id":1,"method":"rpc.discover","params":{}}' | \
  socat - UNIX-CONNECT:/run/mos_volmgrd/mos_volmgrd.sock

Method Reference

Query / Inventory

Method	Params	Description
storage.list_disks	{}	All block devices with type, size, model, serial
storage.list_partitions	{}	All partitions with label, UUID, fstype
storage.list_filesystems	{}	MOS-labeled filesystems (MOSEFI, MOSDATA, etc.)
storage.list_mounts	{}	MOS-managed mount points
storage.list_subvolumes	{}	Btrfs subvolumes on MOSDATA
storage.list_free_space	{"disk": "/dev/sda"}	Unpartitioned gaps on a disk
storage.inventory	{}	All of the above in one call
storage.feasible_topologies	{}	Which topology templates work with current disks

Statistics / Health

Method	Params	Description
storage.usage	{}	Per-filesystem usage (total/used/available/percent)
storage.device_stats	{"mount_point": "..."}	Btrfs device error counters
storage.scrub_status	{"mount_point": "..."}	Scrub running/completed status
storage.fs_usage	{"mount_point": "..."}	Detailed btrfs space breakdown
storage.fs_df	{"mount_point": "..."}	Btrfs data/metadata/system allocation

Volume Management

Method	Params	Description
storage.create_partition	{"disk","name","part_type","size_bytes?"}	Add partition to existing GPT
storage.create_filesystem	{"device","fstype","label"}	Format a partition
storage.mount	{"source","target","fstype","options"}	Mount a filesystem
storage.unmount	{"path": "..."}	Unmount a path

Subvolume Management

Method	Params	Description
storage.create_subvolume	{"name": "..."}	Create btrfs subvolume
storage.delete_subvolume	{"name": "..."}	Delete subvolume (not core ones)
storage.snapshot_subvolume	{"source","snapshot"}	Read-only snapshot
storage.set_quota	{"name","max_bytes"}	Set qgroup quota

Filesystem Utilities

Method	Params	Description
storage.scrub_start	{"mount_point": "..."}	Start integrity scrub
storage.balance_start	{"mount_point": "..."}	Start data rebalance
storage.balance_status	{"mount_point": "..."}	Query balance progress
storage.defrag	{"path": "..."}	Recursive defragmentation
storage.resize_max	{"mount_point": "..."}	Grow filesystem to device size

Discovery

Method	Params	Description
rpc.discover	{}	OpenRPC service document

Error Codes

Code	Meaning
-32700	Parse error (malformed JSON)
-32600	Invalid request
-32601	Method not found
-32602	Invalid params
-32603	Internal error
-32000	Storage operation failed
-32001	Validation error
-32002	Device not found

Partition Layout (mos_sysvol)

When mos_sysvol initializes a disk, it creates a fixed 5-partition GPT layout:

#	GPT Name	Size	Filesystem	Label
1	mosbios	1 MB	(raw)	—
2	mosefi	100 MB	FAT32	MOSEFI
3	mosboot	1 GB	ext4	MOSBOOT
4	mosswap	N GB	swap	MOSSWAP
5	mosdata	N GB	btrfs	MOSDATA

On dual-disk systems, both disks get this layout. mdadm RAID1 mirrors are created for the EFI (metadata v0.90) and boot (metadata v1.2) partitions. The data partitions use btrfs RAID1.

Btrfs subvolumes on MOSDATA: system, etc, modules, vm-meta, mounted at /var/cache/<name>.

Topology Templates

The daemon knows about these storage layout patterns for feasibility analysis:

Topology	Min Disks	Requirements	Description
BtrfsSingle	1	—	Single disk, btrfs data
BcachefsSingle	1	—	Single disk, bcachefs data
DualIndependent	2	—	Independent btrfs per disk
SsdHddBcachefs	2	SSD + HDD	SSD cache + HDD backing via bcachefs
Bcachefs2Copy	2	—	Multi-device bcachefs with 2 replicas
BtrfsRaid1	2	—	Mirrored btrfs across 2 disks

Query feasibility with mos-volmgr topologies.

Design Decisions

• ADR-001 — Workspace consolidation from mos_sysvol + mos_storage
• ADR-002 — Daemon redesign as query/management service
• ADR-003 — JSON-RPC 2.0 over Unix socket

License

Apache-2.0