Hi,
What are those steps to expand or shrink an btrfs over lvm?

-extend
lvextend --resizefs -L new_value /dev/mapper/y-y
-shrink
lvreduce --resizefs -L 400M /dev/mapper/y-y

Thank you

I love using LVM with EXT4. I see no reason to use LVM with BTRFS.

If you must (and I am VERY curious why you would) I would look for two things: 1) how to do it with LVM and EXT4, then 2) how to do it with btrfs. You are going to be replacing the EXT4 steps with the BTRFS commands.

An article like this might help: https://unix.stackexchange.com/quest...cal-lvm-volume where someone else addressed a very similar question.

Madness.
btrfs is not (just) a filesystem, it is more a data manager - aka LVM+ext4 say or zfs. Putting btrfs on top of another block device layer makes no sense. Unless you have a very specific edge case - in which case let's hear about it.

How did you reach this configuration ?. I'm guessing an in-place conversion from ext to btrfs - if so, why ?.

Edit: btrfs wiki is here.

hi,
btrfs device usage /u
/dev/sdc, ID: 1
Device size: 20.00GiB
Data,single: 8.00MiB
Metadata,DUP: 2.00GiB
System,DUP: 16.00MiB
Unallocated: 17.98GiB

/dev/sde, ID: 2
Device size: 20.00GiB
Unallocated: 20.00GiB
Initially before to add /dev/sde
df -h /u
Filesystem Size Used Avail Use% Mounted on
/dev/sdc 20G 3.8M 18G 1% /u

After i added /dev/sde
df -h /u
Filesystem Size Used Avail Use% Mounted on
/dev/sdc 40G 3.8M 38G 1% /u

No need to expand /u.Give me example when i need to extend filesystem?Only if i shrunk first this filesystem?

then
btrfs filesystem resize 9g /u
Resize '/u' of '9g'
linux-l9l8:~ # df -h /u
Filesystem Size Used Avail Use% Mounted on
/dev/sdc 29G 3.8M 27G 1% /u

Why i have the size reduced at 29G?

btrfs fi sh /u
Label: none uuid: bed671b0-9e2b-47dd-b864-75f7cdfef0f0
Total devices 2 FS bytes used 416.00KiB
devid 1 size 9.00GiB used 2.02GiB path /dev/sdc
devid 2 size 20.00GiB used 0.00B path /dev/sde
I see that only filesystem on /dev/sdc is reduced to 9g.
if now
btrfs filesystem resize -9g /u
btrfs filesystem resize: invalid option -- '9'
usage: btrfs filesystem resize [devid:][+/-]<newsize>[kKmMgGtTpPeE]|[devid:]max <path>

Resize a filesystem

If 'max' is passed, the filesystem will occupy all available space
on the device 'devid'.
[kK] means KiB, which denotes 1KiB = 1024B, 1MiB = 1024KiB, etc.
why this?

hi,
https://www.tecmint.com/create-btrfs...stem-in-linux/

it seems that it is not madness to have lvm under brtfs.

LVM is never madness, just needless complexity for many home users small machines.