doc/udev_assembly.txt

Automatic device assembly by udev
=================================

We want to asynchronously assemble and activate devices as their components
become available. Eventually, the complete storage stack should be covered,
including: multipath, cryptsetup, LVM, mdadm. Each of these can be addressed
more or less separately.

The general plan of action is to simply provide udev rules for each of the
device "type": for MD component devices, PVs, LUKS/crypto volumes and for
multipathed SCSI devices. There's no compelling reason to have a daemon do these
things: all systems that actually need to assemble multiple devices into a
single entity already either support incremental assembly or will do so shortly.

Whenever in this document we talk about udev rules, these may include helper
programs that implement a multi-step process. In many cases, it can be expected
that the functionality can be implemented in couple lines of shell (or couple
hundred of C).

Multipath
---------

For multipath, we will need to rely on SCSI IDs for now, until we have a better
scheme of things, since multipath devices can't be identified until the second
path appears, and unfortunately we need to decide whether a device is multipath
when the *first* path appears. Anyway, the multipath folks need to sort this
out, but it shouldn't bee too hard. Just bring up multipathing on anything that
appears and is set up for multipathing.

LVM
---

For LVM, the crucial piece of the puzzle is lvmetad, which allows us to build up
VGs from PVs as they appear, and at the same time collect information on what is
already available. A command, pvscan --cache is expected to be used to
implement udev rules. It is relatively easy to make this command print out a
list of VGs (and possibly LVs) that have been made available by adding any
particular device to the set of visible devices. In othe words, udev says "hey,
/dev/sdb just appeared", calls pvscan --cache, which talks to lvmetad, which
says "cool, that makes vg0 complete". Pvscan takes this info and prints it out,
and the udev rule can then somehow decide whether anything needs to be done
about this "vg0". Presumably a table of devices that need to be activated
automatically is made available somewhere in /etc (probably just a simple list
of volume groups or logical volumes, given by name or UUID, globbing
possible). The udev rule can then consult this file.

Cryptsetup
----------

This may be the trickiest of the lot: the obvious hurdle here is that crypto
volumes need to somehow obtain a key (passphrase, physical token or such),
meaning there is interactivity involved. On the upside, dm-crypt is a 1:1
system: one encrypted device results in one decrypted device, so no assembly or
notification needs to be done. While interactivity is a challenge, there are at
least partial solutions around. (TODO: Milan should probably elaborate here.)

(For LUKS devices, these can probably be detected automatically. I suppose that
non-LUKS devices can be looked up in crypttab by the rule, to decide what is the
appropriate action to take.)

MD
--

Fortunately, MD (namely mdadm) already comes with a mechanism for incremental
assembly (mdadm -I or such). We can assume that this fits with the rest of stack
nicely.


Filesystem &c. discovery
========================

Considering other requirements that exist for storage systems (namely
large-scale storage deployments), it is absolutely not feasible to have the
system hunt automatically for filesystems based on their UUIDs. In a number of
cases, this could mean activating tens of thousands of volumes. On small
systems, asking for all volumes to be brought up automatically is probably the
best route anyway, and once all storage devices are activated, scanning for
filesystems is no different from today.

In effect, no action is required on this count: only filesystems that are
available on already active devices can be mounted by their UUID. Activating
volumes by naming a filesystem UUID is useless, since to read the UUID the
volume needs to be active first.