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GNU/Linux is a monolithic kernel, meaning that a lot of functionality
is baked into the kernel, including filesystems like `ext4` or `xfs`.
Alternatively, the Hurd's filesystems are in userspace, but our disk
device drivers are baked into the GNU Mach kernel image (via
[[DDE|hurd/dde]]).  With [[rumpdisk|hurd/rump/rumpdisk]], the Hurd can
use SSDs with userspace device drivers! RumpDisk uses `libmachdev` as
a helper library.

`libmachdev` exposes devices to userspace via some Mach `device-*` RPC
calls.  `libmachdev` provides a trivfs that intercepts the
`device_open` RPC, which the `/dev` node uses.  It also fakes a
filesystem node, so you can mount a `netfs` onto it.  You still have
to implement `device_read` and `device_write` yourself, but that code
runs in userspace.  An example of this can be found in
`rumpdisk/block-rump.c`.

If [[serverboot V2|open_issues/serverbootv2]] is written, then
`libmachdev` could be simplified or maybe removed.