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We shouldn't be losing pvscans just because of the fact that the
underlying device (PV) appears and disappears quickly in the system,
otherwise lvmetad may not see the device if it appears again (or it may
still keep the device in cache even it's already gone).
When lvm2-pvscan@.service and lvm2-lvmetad.service are scheduled to be
stopped lvm2-pvscan@.service should be stopped first since pvscan uses
lvmetad.
This is especially important if lvm2-lvmetad.socket is also scheduled to
be stopped as in this case connection requests are suppressed causing
pvscan to fail.
When using filters for the pvscan --cache (the global_filter),
there's a difference between:
pvscan --cache -aay /dev/block/<major>:<minor>
and
pvscan --cache -aay <major>:<minor> (or --major <major> --minor <minor>)
In the first case, we need to be sure to have an exact matching line
in the filter for the device to be used, no aliases are considered
So for example even if we have accept rule for "/dev/sda" present,
this won't apply for "/dev/block/8:0" even though it's the same device!
This is because we're comparing the path used on command line directly
with the path written in the rule.
For the second one, any alias mentioned in the filter will apply
as we're comparing the major and minor pair, not looking at actual
device names - so any alias mentioned in the rules will suffice for
the filtering rule to apply.
For the global_filter to be properly used, we need to call the
second one in the lvm2-pvscan@.service - nobody is able to tell
what value of major:minor the kernel assignes next time, hence
this bug makes the use of global_filter quite unusable!
When using ENV{SYSTEMD_WANTS}=lvm2-pvscan@... to instantiate a service
for lvmetad scan when the new PV appears in the system, the service
is started and executed. However, to track device removal, we need
to bind it (the "BindsTo" systemd directive) to a certain .device
systemd unit.
In default systemd setup, the device is tracked by it's name and
sysfs path (there's normally a sysfs path .device systemd unit for
a device and then the device name .device unit as an alias for it).
Neither of these two is useful for lvmetad update as we need to bind
it to device's <major>:<minor> pair.
The /dev/block/<major>:<minor> is the essential symlink under /dev
that exists for each block device (created by default udev rules
provided by udev directly). So let's use this as an alias for
the device's .device unit as well by means of "ENV{SYSTEMD_ALIAS}"
declaration within udev rules which systemd understands (this will
create a new alias "dev-block-<major>:<minor>.device".
Then we can easily bind the "dev-block-<major>:<minor>" device
systemd unit with instantiated lvm2-pvscan@<major>:<minor>.service.
So once the device is removed from the systemd, the
lvm-pvscan@<major>:<minor>.service executes it's ExecStop action
(which in turn notifies lvmetad about the device being gone).
This completes the udev-systemd-lvmetad interaction then.
The new lvm2-pvscan@.service is responsible for on-demand execution
of "pvscan --cache --activate ay" which causes lvmetad to be
updated and LVM activation done if the VG is complete.
Also, use udev-systemd mechanism to instantiate the job as the
lvm2-pvscan@$devnode.service on each newly appeared PV in the system.
This prevents the background job to be killed (that would happen
if it was directly forked from udev rule - this behaviour is seen
in recent versions of udev with the help of systemd that can track
detached processes - the detached process would still be in the same
cgroup).
To enable this official udev-systemd protocol for instantiating
background jobs, use new --enable-udev-systemd-background-jobs
configure switch (it's disabled by default). This option is highly
recommended wherever systemd is used!
