[linux-lvm] boot volume in activated containing PV does not activate

[Xen]

Quick question.

I have a mirrored LV which I boot from (/boot).

The mirror is within an embedded PV (PV -> LV = PV -> LV = mirror of
other one sitting on other disk).

My initrd activates both the external (outer) PV as well as the root LV.

So:

Collection PV is called "coll" (its VG is called "coll").

Coll contains a PV called "coll/msata-lv" which is an LV that is a PV to
the "msata" volume group that contains the 2 mirrors.

Now at boot (prior to running of systemd) I activate both coll/msata-lv
and msata/root.

This works fine when both mirrors are present.

As a test I removed the primary (SSD) mirror of the root volume (and the
boot volume). Now the system still boots (off of another disk which has
grub on it, but only the grub core and boot image, nothing else, so it
still references my VG) and the root volume still gets activated but the
boot volume doesn't get activated anymore.

What can cause this? It ought to get activated by udev rules.

You remember I patched the udev file to ensure a PV directly on disk
always gets activated but this is not that disk.

Could it be that the msata/boot volume doesn't get activated because the
PV had already been activated in the initrd but only as an LV and not
its volumes?

I mean:

coll/msata-lv is already activated and it contains (is) the PV that has
parts of both msata/root and msata/boot. These are both half of a mirror
that is not present at present (at current). msata/root is explicitly
activated in the initrd, but msata/boot isn't.

As the system starts msata/boot is not activated and the system enters
emergency mode. A simple vgchange -ay msata does the job and msata/boot
gets activated now.

Therefore a simple vgchange -ay at boot (using .e.g. /etc/init.d/lvm2)
also solves it.

[Linda A. Walsh]

Post by Xen
Now at boot (prior to running of systemd) I activate both
coll/msata-lv and msata/root.

---
Systemd requires that it be PID 1 (the first process to run
on your system). I'm told that it won't run unless that is true. So
how can you do anything before systemd has been run?

Post by Xen
This works fine when both mirrors are present.
As a test I removed the primary (SSD) mirror of the root volume (and
the boot volume). Now the system still boots (off of another disk
which has grub on it, but only the grub core and boot image, nothing
else, so it still references my VG) and the root volume still gets
activated but the boot volume doesn't get activated anymore.
What can cause this? It ought to get activated by udev rules.
You remember I patched the udev file to ensure a PV directly on disk
always gets activated but this is not that disk.
Could it be that the msata/boot volume doesn't get activated because
the PV had already been activated in the initrd but only as an LV and
not its volumes?

Sounds very similar to opensuse's requirement that /usr be on
the same partition as root -- if not, then you have to boot using
a ramdisk, which mounts /usr, and then does the real system boot, so
of course, booting directly from disk (which is what my machine does)
is not supported. I also mount "/usr" as a first step in my boot
process, but that disallows a systemd boot, which some define as
systemd being pid 1.

I'm told systemd has changed the meaning of 'fstab' to not
be a table of disks to mount, but to be a table of disks to keep mounted
(even if umount'd by an administrator). Since udev's functionality was
incorporated into systemd, might it not be doing something similar --
trying to maintain the boot-state in effect before you added the mirror?

[Xen]

Post by Linda A. Walsh

Post by Xen
Now at boot (prior to running of systemd) I activate both
coll/msata-lv and msata/root.

---
Systemd requires that it be PID 1 (the first process to run
on your system). I'm told that it won't run unless that is true. So
how can you do anything before systemd has been run?

Hey lover :p.

Because I do it in the initrd. I'm not sure how much of that is "handed
over" to systemd but it makes a big difference. For example,
pre-activating one half of my (embedded) mirrors was the key to making
sure the entire mirror is activated properly upon boot.

If not, I would have to run something like vgchange --refresh or
lvchange --refresh or something of the kind.

Post by Linda A. Walsh

Post by Xen
This works fine when both mirrors are present.
As a test I removed the primary (SSD) mirror of the root volume (and
the boot volume). Now the system still boots (off of another disk
which has grub on it, but only the grub core and boot image, nothing
else, so it still references my VG) and the root volume still gets
activated but the boot volume doesn't get activated anymore.
What can cause this? It ought to get activated by udev rules.
You remember I patched the udev file to ensure a PV directly on disk
always gets activated but this is not that disk.
Could it be that the msata/boot volume doesn't get activated because
the PV had already been activated in the initrd but only as an LV and
not its volumes?

Sounds very similar to opensuse's requirement that /usr be on
the same partition as root -- if not, then you have to boot using
a ramdisk, which mounts /usr, and then does the real system boot, so
of course, booting directly from disk (which is what my machine does)
is not supported. I also mount "/usr" as a first step in my boot
process, but that disallows a systemd boot, which some define as
systemd being pid 1.

So you skip an initrd right(or initramfs). My Ubuntu thing just mounts
itself as /, then pivots to the real root after activating some stuff.

On Ubuntu (and Debian) you create hooks and scripts and then they just
get embedded into initramfs and that just gets loaded prior to systemd
doing anything.

It was said that LVM (or actually, SystemD and UDev) just processes all
things again; all devices all passed through the udev system once more,
and so everything is getting activated or at least processed (creates a
udev trigger for something to happen).

The strange thing was -- and I was just a bit confused just a moment
about about a similar Grub question I asked somewhere -- that...

while booting from my regular msata thing that had the first half of the
mirrors, and while pre-activating the outer-embedding PV (and its LV)
that is used for that second half, this was enough to get the boot
volume completely activated without explicitly doing so.

Meaning, the second PV to "msata" volume group was now activated and
this PV contains both the second half of msata/root and the second half
of msata/boot. As a consequence, when boot is activated by systemd, it
finds the volume (PV) just fine.

But now for some strange reason it does not, so now that I remove the
msata disk, the PV for it (the second half that is still there) is still
getting activated but now that the first half is not found, and
triggered, so to speak...

the second half suddenly does not prompt a systemd activation anymore.
Actually systemd is no part of it at all, it is just the udev rule that
rules the pvchange.

So *apparently* there is no udev rule being run on the second-half PV
(that contains both second-halves) and the *root* one has already been
activated by my initrd (ramfs) so there's no problem there.

The boot one (which is just one half at this point) does not get
activated at all. Which leads me to leave the udev rule misses it.

Post by Linda A. Walsh
I'm told systemd has changed the meaning of 'fstab' to not
be a table of disks to mount, but to be a table of disks to keep mounted
(even if umount'd by an administrator). Since udev's functionality was
incorporated into systemd, might it not be doing something similar --
trying to maintain the boot-state in effect before you added the mirror?

I don't think so. At this point there is no mirror because I am only
providing one half. I am pretty sure the bootstate is maintained if I
were to preactivate the boot LV in the initrd(ramfs).

I am sorry for not having responded to your email of last January. I
guess I'm living in a slow-time bubble. There was a reason I couldn't
respond, I don't remember what it was.

Seems like 3 days ago for me. Nothing much happens except for terrible
things. Everything moves as fast as my feet can take me, which is not
very fast.