Why does Anaconda leave 100GB empty for LVM Thin ?

[David Lehman]

Hi,

On Fri, Aug 27, 2021 at 3:00 PM Sreyan Chakravarty <sreyan32 at gmail.com>
wrote:

> Hi,
>
> I have just installed a new Fedora 33 KDE system.
>
> So this is my layout:
>
>
> NAME                                          MAJ:MIN RM   SIZE RO TYPE
>  MOUNTPOINT
> sda                                             8:0    0 931.5G  0 disk
> ├─sda1                                          8:1    0   512M  0 part
>  /boot/efi
> ├─sda2                                          8:2    0     1G  0 part
>  /boot
> └─sda3                                          8:3    0   930G  0 part
>  └─luks-aee439f3-528c-4481-ab09-622304e727bc 253:0    0   930G  0 crypt
>    ├─vgfedora-pool00_tmeta                   253:1    0   424M  0 lvm
>    │ └─vgfedora-pool00-tpool                 253:3    0 829.2G  0 lvm
>    │   ├─vgfedora-fedora                     253:4    0 829.2G  0 lvm   /
>    │   └─vgfedora-pool00                     253:5    0 829.2G  1 lvm
>    └─vgfedora-pool00_tdata                   253:2    0 829.2G  0 lvm
>      └─vgfedora-pool00-tpool                 253:3    0 829.2G  0 lvm
>        ├─vgfedora-fedora                     253:4    0 829.2G  0 lvm   /
>        └─vgfedora-pool00                     253:5    0 829.2G  1 lvm
>
> The same thing as above in pastebin: https://pastebin.com/raw/xwXngMRh
>
> If you notice that /dev/sda3 is of size 930G, but the volume group is only
> 829.2 G.
>
> Why is that ? I have selected the size of the volume group as "Large as
> possible", but still 100G is left out.
>
> Is this some kind of recommended setting for thin-LVMs ?
>
> I am selecting Thin-LVM with LUKS2 during the install.
>
> Can someone help me understand why this is happening and how I can fix it
> ?
>

There are some circumstances under which LVM may need additional space for
thin pool metadata. When we added support for thin provisioning they
strongly recommended that we reserve a portion of the volume group space
for this purpose.Initially it was 20% of the pool size, but the calculation
may be more nuanced now. I wouldn't consider removing this bit of free
space "fixing" anything unless you plan to actively monitor your system to
ensure that your thin pool doesn't run out of space for metadata.


>
> Will making a custom kickstart help here ?
>

Maybe... It has been a while since I worked on this code, but you _might_
be able to override this in kickstart using `--reserved-space` or
`--reserved-percent` options to the `volgroup` command.

HTH,
David


>
> --
> Regards,
> Sreyan Chakravarty
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