Volume Management and Auto-mount

What is it?

Volume management decides which filesystem sits on each partition and which drive letter it gets. Today one boot-time function does all of it with fixed rules: C: is the active system slot, BlackBox is X:, and other FAT32 and NTFS partitions take D: onward. This roadmap replaces that function with a probe chain and letter allocator, then adds USB hot-plug mounting, safe removal, mount and umount commands, the Win32 volume queries and volume control codes. None of its eight sections is complete.

How does it work?

Boot order. boot_storage.c registers the block devices, then calls vfs_init(), partition_scan_all() and partition_mount_filesystems() with the active A/B slot. vfs_init() in vfs.c creates a table of 26 drive letters (VFS_MAX_DRIVES in vfs.h).

Probing. probe_filesystem() in partition.c reads the first sector of a partition and tries IXFS, then NTFS, then FAT32, then reads sector 2 for the ext2 magic. The result is one of the PART_FS_* codes in partition.h. A partition that matches nothing is skipped without a message, and an ext2 or ext4 partition is recognised but never mounted.

Letter rules. partition_mount_filesystems() applies them in order:

• C: is the IXFS root of the active A/B slot. If the slot's root cannot be mounted, or the boot disk is ambiguous (duplicate GUIDs or several A/B disks), C: is left unmounted on purpose and the bootloader rolls back next boot (A/B Boot and Rollback).
• The EFI System Partition, the recovery partition and the other A/B slot get no letter.
• A FAT32 partition whose GPT name is BlackBox becomes X: (BlackBox Service Partition).
• Every other FAT32 or NTFS partition takes the next letter from D:, skipping X:, in scan order.

Letters are not stored anywhere, so the same disk can land on a different letter if the scan order changes.

Removal and hot-plug. vfs_unmount() only clears the drive slot; it does not flush or close open files. AHCI port hot-plug is detected in ahci_hotplug.c, but a new disk is not probed or mounted, and a removed one is flushed at the port but never unmounted. A USB stick is only mounted if it was present at boot (Storage Controllers and Removable Media).

Volume queries. NtQueryVolumeInformationFile in nt_file.c returns fixed values for every drive: the label Impossible, serial 0x494D5053, filesystem IXFS and a fixed size. NtFsControlFile returns STATUS_INVALID_DEVICE_REQUEST for every control code.

flowchart TD
    B[boot_storage] --> S[partition_scan_all]
    S --> P[probe: IXFS, NTFS, FAT32, ext2]
    P --> C{partition role}
    C -->|active A/B slot| Cc[C:]
    C -->|GPT name BlackBox| X[X:]
    C -->|FAT32 or NTFS| D[next letter from D:, skip X:]
    C -->|ESP, recovery, other slot| N[no letter]
    C -->|ext2 or unknown| U[not mounted]

What are its interfaces?

How do I use it?

Boot with extra disks attached and read the serial log. The lines to look for:

VFS initialized (26 drive letters A:\ - Z:\)
A/B: mounted slot <n> as C:
BlackBox partition mounted as X:\
Mounted NTFS volume on drive D: (<sectors> sectors)

There is no command to list, mount or unmount volumes yet. The VFS suites in test_vfs.c (make test-fs) check the drive-letter range and file round trips, but not the letter assignment rules.

What is not implemented yet?

• A probe chain and letter allocator recorded in the Registry (Filesystem Probe Chain + Drive-Letter Assignment), and the boot mount rewrite onto it (Boot Mount Sequence Rewrite).
• USB hot-plug mounting (USB Hot-Plug Volume Arrival) and safe removal (USB Safe Removal).
• mount and umount commands (Manual mount / umount Shell Commands).
• Real Win32 volume answers such as GetLogicalDrives and GetVolumeInformationW (Win32 Volume Query APIs).
• Optical discs (Optical Drive Handling) and volume control codes such as dismount and lock (Volume Control Ioctls).

How does it compare with Windows 11 and Linux?

Windows 11's mount manager (mountmgr.sys) assigns and remembers letters, mounts hot-plugged disks and offers Safely Remove Hardware; Linux mounts through udev, udisks2 and /etc/fstab, with mount and umount commands. Impossible OS mounts what is present at boot by fixed rules and has no hot-plug mount, removal or remembered letters yet.

See also

• Volume management roadmap
• A/B Boot and Rollback
• BlackBox Service Partition
• Storage Controllers and Removable Media
• Optical Media
• Storage