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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.