[linux-lvm] what is free space in LVM

[linux-lvm] what is free space in LVM

[Angelo Moreschini]

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Hi,

I am new to LVM and I would like to have an exact answer to the question :

what is the criteria that allows to distinguishes the "free space" on the
all the space in a Volume Group?

or (if you prefer)...

how the command pvscan does  to obtain information about the free space in
LVM ?

------------
According to my current knowledge, I know that the definition of "free
space" concerns with with the set of blocks for which, in the FAT, is not  r
egistered a significant link to access a file.

But I think that in the context of LVM there should be a different criterion to
define the "free space" inside a Volume Group.
----------

Thank you

Regards

Angelo

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Re: [linux-lvm] what is free space in LVM

[Mauricio Tavares]

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On Jul 13, 2015 9:24 AM, "Angelo Moreschini" <mrangelo.fedora@gmail.com>
wrote:
>
> Hi,
>
> I am new to LVM and I would like to have an exact answer to the question :
>
> what is the criteria that allows to distinguishes the "free space" on the
all the space in a Volume Group?
>
> or (if you prefer)...
>
> how the command pvscan does  to obtain information about the free space
in LVM ?
>
      This may sound like a naive reply but I always thought free space in
a VG is space not allocated to its LVs.

> ------------
> According to my current knowledge, I know that the definition of "free
space" concerns with with the set of blocks for which, in the FAT, is not
registered a significant link to access a file.
>
> But I think that in the context of LVM there should be a different
criterion to define the "free space" inside a Volume Group.
> ----------
>
> Thank you
>
> Regards
>
> Angelo
>
> _______________________________________________
> linux-lvm mailing list
> linux-lvm@redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/

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Re: [linux-lvm] what is free space in LVM

[Zdenek Kabelac]

Dne 10.7.2015 v 11:45 Angelo Moreschini napsal(a):
> Hi,
>
> I am new to LVM and I would like to have an exact answer to the question :
>
> what is the criteria that allows to distinguishes the "free space" on the all
> the space in a Volume Group?
>
> or (if you prefer)...
>
> howthe command pvscan does to obtain information about the free space in LVM ?
>
> ------------
> According to my current knowledge, I know that the definition of "free space"
> concerns with withthe set of blocks for which, in the FAT, is not  registered
> a significant link to access a file.
>
> But I think that in the context of LVM there should be a different criterion
> to define the "free space" insidea Volume Group.
> ----------

lvm2 is NOT filesystem

lvm2 is 'volume manager' - thus  every PV is divided into extents.
And 'extent' could be associated with LV.

So easily - if PV has  32bit #extent count.
(Size of extent is again 32bit number -> in total 64bit address space).

So when you calc all extents in use (being member of any LV),
and subtracts this from number of all available extent
you will get number of 'free' extent.

Note - you put filesystem ON your LV - and then filesystem is managing free 
space within this LV.

Zdenek