[linux-lvm] Usable space in PV

[Timofey Koolin]

Task: calc usable space of PV before create PV and before create
partition for PV.

In my experiments usable space in PV = PVSize / ExtentSize * ExtentSize
with integer operations.
But when Metadata may be large - when LVM have a lot of logical volumes
and it can take more space.

I found in /etc/lvm/lvm.conf parameter pvmetadatasize = 255 secotors
(default).
What will if metadata will grow to more then 255 sectors?

[Zdenek Kabelac]

Task: calc usable space of PV before create PV and before create partition for
PV.
In my experiments usable space in PV = PVSize / ExtentSize * ExtentSize with
integer operations.
But when Metadata may be large - when LVM have a lot of logical volumes and it
can take more space.
I found in /etc/lvm/lvm.conf parameter pvmetadatasize = 255 secotors (default).
What will if metadata will grow to more then 255 sectors?

If you plan to host lots of LVs within a single VG (in range of hundreds)
use i.e. 'pvcreate --metadatasize 5M'
(i.e. ~7000 linear single segment LVs takes around 2.2MB and you need space
for at least 2 metadatas to fit in)

What I'm not quite sure is your 'task' - what's the purpose ?
Do you want to do some 'sector' precise emulation of what lvm2 is doing by
watching resuls of couple command with just subset of it's options ?

Isn't better to read man pages ?

Zdenek

[Timofey Koolin]

Task - problem.
I make universal extender of filesystem (for example extend fs in
virtual computer after resize disk or add space to filesystem in hosting
server, etc.). Before extend volume group and filesystem I want to make
plan of extends. For plan I need to know how many bytes it can add to VG
from disk/partition. I know size of disk, partition, free space - and I
have to know free space after create PV on free space of disk.
I don't know how many LV will be on target system (system, which run
fsextender), I don't know fragmentation of it and etc.
Is any way to know how many space take metadata and how many space will
be usable?

Post by Zdenek Kabelac

Task: calc usable space of PV before create PV and before create partition for
PV.
In my experiments usable space in PV = PVSize / ExtentSize *
ExtentSize with
integer operations.
But when Metadata may be large - when LVM have a lot of logical volumes and it
can take more space.
I found in /etc/lvm/lvm.conf parameter pvmetadatasize = 255 secotors (default).
What will if metadata will grow to more then 255 sectors?

If you plan to host lots of LVs within a single VG (in range of hundreds)
use i.e. 'pvcreate --metadatasize 5M'
(i.e. ~7000 linear single segment LVs takes around 2.2MB and you need
space for at least 2 metadatas to fit in)
What I'm not quite sure is your 'task' - what's the purpose ?
Do you want to do some 'sector' precise emulation of what lvm2 is
doing by watching resuls of couple command with just subset of it's
options ?
Isn't better to read man pages ?
Zdenek
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