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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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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 othe words, udev says "hey,
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/dev/sdb just appeared", calls pvscan --lvmetad, which talks to lvmetad, which
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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