[lug] lvm and physical volume worries

[karl horlen]

I keep seeing an occasional warning in LVM posts and
documentation that you should NOT span LVM across
multiple physical volumes because if one PV goes down
you lose the whole LVM.

I just want to make sure I understand this.

Are they talking about SPANNING the entire VG (volume
group) or just an individual LV (logical volume) in a
VG over multiple PVs?  

** BTW, when I use PV here and other spots below, I
mean a PV that is an entire disk partition and not
just a smaller partition of a disk.  This implies that
multiple PVs live across MULTIPLE PHYSICAL disks and
not just multiple partitions on ONE physical disk.  I
hope that's clear.

And

Are they talking about LOSING the entire VG (volume
group) or just an individual LV (logical volume) in a
VG spread over multiple PVs?

The reason I ask is because assuming a VG consists of
multiple PVs, there is no way that I know of that you
can tell the LV which PV[s] to actually use.  And
without that ability, it would seem that even if your
LV was smaller than the smallest PV, there would not
be any guarantee that the LV filesystem would be
written entirely on one PV versus spread across
multiple PVs.  Or that multiple LVs live on a single
PV.

So the way I'm reading this is that the minute you
lose any PV in a VG you lose the entire VG.  Can
someone confirm this?

If that's the case, then LVM is severely limited in
multi PV configurations.  Or at least it means that
your VG should never consist of more than one PV (or
all your PVs should be partitions on one disk) if you
care about the integrity of your data in multi PV
setup.  Would that be a pretty good rule of thumb?

I imagine you can create  backups of your VG that
consists of multiple PVs in the event it goes down. 
But the restore is probably going to be a lot of work
and at that point the extra complexity of a multi PV
VG probably isn't worth it.

In this case, the best bet might be to  mirror the VG
(never greater than one PV) on a RAID1.

Does this sound like a good plan?
Does anybody have any better recommendations?

thanks


 
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