Module snapshot_cow 

Block-level copy-on-write for sandbox rootfs via dm-snapshot.

Instead of copying the full rootfs ext4 image for every sandbox, CowManager creates a dm-snapshot backed by a sparse COW file. The template image is shared read-only across all sandboxes that use the same rootfs; only written blocks consume disk space.

Requires CONFIG_DM_SNAPSHOT=y in the kernel, a dmsetup binary at one of CowOptions::dmsetup_candidates, and loop-device tooling supplied through the BlockTools seam (BusyboxBlockTools is the reference: the System VM’s busybox applets; UtilLinuxBlockTools is the stock distro’s /sbin userland). PATH is never searched — neither the guest nor a node agent has a meaningful one, so both implementations take absolute paths.

Structs

BusyboxBlockTools

BlockTools over busybox applets — the System VM’s userland.

CowHandle

Per-sandbox CoW state. Stored in SandboxInstance for cleanup.

CowManager

Manages template loop devices and per-sandbox dm-snapshot lifecycle.

CowOptions

What a CowManager needs from its environment.

UtilLinuxBlockTools

BlockTools over util-linux, the block tooling a stock distro ships.

Constants

COW_FILE_PREFIX

File-name prefix of a sandbox’s copy-on-write overlay, under cow_dir.

COW_FILE_SUFFIX

File-name suffix of a sandbox’s copy-on-write overlay. Optional when reading one back: reconcile_stale accepts a name without it.

DM_NAME_PREFIX

Device-mapper name prefix for sandbox snapshots.

Traits

BlockTools

Loop-device and block-size operations, supplied by the composer.

Functions

device_major_minor

The (major, minor) device numbers of the block device at path.

mknod_blkdev

Create a block device node at node_path for (major, minor), mode 0600.