Re: Reg LVM PE Size and any limitations out of it

Dne 29. 05. 25 v 19:06 Lakshmi Narasimhan Sundararajan napsal(a):
> Hi Team,
> A very good day to you.
> 
> During vgcreation there exists a field '-s|--physicalextentsize'.
>  From the man pages it calls out as extent size of PVs in VG.
> As a reference on a vg already created, I see that it is called out as
> 4MiB. [see below]


Size of volume group is defined as 32bit unsigned extent size x  32bit 
unsigned extent count.

The size is internally using 512b units and we can create a volume group
capable of creating  ~8EiB volumes -  user just needs to set 4G extent size 
(so then user is restricted in created LV size ->  4GLV, 8GLV, .... )

> My questions are as follows:
> 1) Is this dynamically computed based on the installed PV during VG creation?
> Given this cannot be changed after VG creation, what would happen if

ATM 4MiB is applied by default.
But since the storage sizes grow so fast - we may consider for really large 
volumes to pick bigger extent size (maybe 1GiB) in future.

(Also if you is planning to use  lots of LV - he needs to create bigger 
metadata area - as with default ~1MiB we can handle around 1000 LVs...)


> the pool were to grow or even the disks were to grow (think san
> volumes). would vg limit itself to some capacity limitations above
> which it cannot grow, leading to my second follow up question.
> 2) Is there any limit to the max number of PE in a VG?

Yep max PE count is  32bit unsigned  aka  4G


> It is not called out in any man pages, but an internet search gave me
> a max of 16M extents in a VG. So here I am asking for clarification,

With 4M extent size it goes like this:

Max addressable VG space is  4M x 4G extents  -> ~ 16PiB

If you need to use bigger volumes - you need to use bigger extent size as with 
4MiB extent size you can create at most 16PiB sized LV.


Regards

Zdenek