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87 lines
3.2 KiB
Plaintext
87 lines
3.2 KiB
Plaintext
Q: Why should lvmetad cache foreign VGs?
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A: It's the most useful behavior in the "steady state".
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How to arrive at that conclusion.
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Four code configurations to consider, each in two different circumstances.
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configurations:
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1. lvm not using lvmetad
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2. lvm using lvmetad and lvmlockd
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3. lvm using lvmetad, and lvmetad does not cache foreign VGs
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(Not currently implemented.)
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4. lvm using lvmetad, and lvmetad caches foreign VGs
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circumstances:
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A. steady state: PVs are not added or removed to/from foreign VGs
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B. transient state: PVs are added or removed to/from foreign VGs
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combinations:
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1.A. A PV is correctly shown in the foreign VG.
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1.B. A PV is correctly shown in the foreign VG.
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The most accurate representation, at the cost of always scanning disks.
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2.A. A PV is correctly shown in the foreign VG.
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2.B. A PV is correctly shown in the foreign VG.
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The most accurate representation, at the cost of using lvmlockd.
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3.A. A PV in a foreign VG is shown as unused.
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3.B. A PV in a foreign VG is shown as unused.
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If lvmetad ignores foreign VGs and does not cache them, the PVs in the
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foreign VGs appear to be unused. This largely defeats the purpose of
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system_id, which is meant to treat VGs/PVs as foreign instead of free
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(albeit imperfectly, see below.)
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4.A. A PV is correctly shown in the foreign VG.
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4.B. A PV is not correctly shown in the foreign VG.
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This avoids the cost of always scanning disks, and avoids the cost of
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using lvmlockd. The steady state 4.A. is an improvement over the steady
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state 3.A. When the steady state is the common case, this is a big
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advantage. When the steady state is *not* the common case, the foreign VG
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concept is not as useful (if shared devices are this dynamic, lvmlockd
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should be considered.)
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The limitations related to the transient state 4.B. are explained in
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lvmsystemid(7), along with how to handle it. The specific inaccuracies
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possible in 4.B. are:
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. PV is shown as belonging to a foreign VG, but is actually unused.
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. PV is shown as unused, but actually belongs to a foreign VG.
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To resolve the inaccuracies in the transient state (4.B.), and return the
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system to an accurate steady state (4.A.), the disks need to be scanned,
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which updates lvmetad. The scanning/updating is a manual step, i.e.
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running 'pvscan --cache', which by definition scans disks and updates
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lvmetad.
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--
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The --foreign command line option for report/display commands
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(vgs/lvs/pvs/vgdisplay/lvdisplay/pvdisplay) is not directly related to
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whether or not lvmetad caches foreign VGs.
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By default, foreign VGs are silently ignored and not printed by these
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commands. However, when the --foreign option is used, these commands do
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produce output about foreign VGs.
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(When --foreign is not used, and the command specifically requests a
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foreign VG by name, an error is produced about not accessing foreign VGs,
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and the foreign VG is not displayed.)
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The decision to report/display foreign VGs or not is independent of
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whether lvmetad is caching those VGs. When lvmetad is caching the foreign
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VG, a report/display command run with --foreign will scan disks to read
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the foreign VG and give the most up to date version of it (the copy of the
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foreign VG in lvmetad may be out of date due to changes to the VG by the
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foreign host.)
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