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84 lines
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84 lines
4.0 KiB
Plaintext
Automatic device assembly by udev
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=================================
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We want to asynchronously assemble and activate devices as their components
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become available. Eventually, the complete storage stack should be covered,
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including: multipath, cryptsetup, LVM, mdadm. Each of these can be addressed
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more or less separately.
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The general plan of action is to simply provide udev rules for each of the
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device "type": for MD component devices, PVs, LUKS/crypto volumes and for
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multipathed SCSI devices. There's no compelling reason to have a daemon do these
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things: all systems that actually need to assemble multiple devices into a
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single entity already either support incremental assembly or will do so shortly.
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Whenever in this document we talk about udev rules, these may include helper
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programs that implement a multi-step process. In many cases, it can be expected
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that the functionality can be implemented in couple lines of shell (or couple
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hundred of C).
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Multipath
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---------
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For multipath, we will need to rely on SCSI IDs for now, until we have a better
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scheme of things, since multipath devices can't be identified until the second
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path appears, and unfortunately we need to decide whether a device is multipath
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when the *first* path appears. Anyway, the multipath folks need to sort this
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out, but it shouldn't bee too hard. Just bring up multipathing on anything that
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appears and is set up for multipathing.
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LVM
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---
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For LVM, the crucial piece of the puzzle is lvmetad, which allows us to build up
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VGs from PVs as they appear, and at the same time collect information on what is
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already available. A command, pvscan --cache is expected to be used to
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implement udev rules. It is relatively easy to make this command print out a
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list of VGs (and possibly LVs) that have been made available by adding any
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particular device to the set of visible devices. In other words, udev says "hey,
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/dev/sdb just appeared", calls pvscan --cache, which talks to lvmetad, which
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says "cool, that makes vg0 complete". Pvscan takes this info and prints it out,
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and the udev rule can then somehow decide whether anything needs to be done
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about this "vg0". Presumably a table of devices that need to be activated
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automatically is made available somewhere in /etc (probably just a simple list
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of volume groups or logical volumes, given by name or UUID, globbing
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possible). The udev rule can then consult this file.
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Cryptsetup
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----------
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This may be the trickiest of the lot: the obvious hurdle here is that crypto
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volumes need to somehow obtain a key (passphrase, physical token or such),
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meaning there is interactivity involved. On the upside, dm-crypt is a 1:1
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system: one encrypted device results in one decrypted device, so no assembly or
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notification needs to be done. While interactivity is a challenge, there are at
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least partial solutions around. (TODO: Milan should probably elaborate here.)
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(For LUKS devices, these can probably be detected automatically. I suppose that
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non-LUKS devices can be looked up in crypttab by the rule, to decide what is the
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appropriate action to take.)
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MD
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--
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Fortunately, MD (namely mdadm) already comes with a mechanism for incremental
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assembly (mdadm -I or such). We can assume that this fits with the rest of stack
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nicely.
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Filesystem &c. discovery
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========================
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Considering other requirements that exist for storage systems (namely
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large-scale storage deployments), it is absolutely not feasible to have the
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system hunt automatically for filesystems based on their UUIDs. In a number of
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cases, this could mean activating tens of thousands of volumes. On small
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systems, asking for all volumes to be brought up automatically is probably the
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best route anyway, and once all storage devices are activated, scanning for
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filesystems is no different from today.
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In effect, no action is required on this count: only filesystems that are
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available on already active devices can be mounted by their UUID. Activating
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volumes by naming a filesystem UUID is useless, since to read the UUID the
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volume needs to be active first.
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