[lug] lvm and physical volume worries

[Dan Ferris]

I've spanned across the following and had no problems:

-2 Firewire drives
-a 30GB partition and a 120 GB drive
-1 TB hardware raid arrays
-Linux software raids.

If you want tolerance, then make sure the PV is fault tolerant.  LVM 
lives above RAID and it doesn't care what the hardware looks like.  If 
you had a SAN, you could span across SAN volumes.  It doesn't matter.

It is funny when LVM looses a volume though, the filesystem goes utterly 
wacko in a way that is hard to describe heh heh heh.

If you want an example...

My computer here has 2 80 GB Hitachi drives in it.  They are set up in a 
Linux software RAID 0 (striping).  I am using LVM on the stripe to get 
my volumes.  It was easier to do it that way than to make 5 different 
stripe arrays for each disk partition.

Dan

karl horlen wrote:
> 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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