root@test:~# df -h

/dev/mapper/cachedev_0   86G   80M   86G   1% /volume1

root@test:~# synopkg onoffall stop
Stop SynoFinder
Stop SupportService
Stop StorageManager
Stop SecureSignIn
Stop ScsiTarget
Stop SMBService
Stop QuickConnect
Stop Python2
Stop OAuthService
Stop HybridShare
Stop FileStation
Stop ActiveInsight
stop all packages successfully
 sftp.sh  synomount.sh  tftp_serv
root@test:/usr/syno/etc.defaults/rc.sysv# ./synomount.sh stop
Remove udf.ko ...
Remove isofs.ko ...
root@test:/usr/syno/etc.defaults/rc.sysv# ./a stop
apparmor.sh  avahi.sh
root@test:/usr/syno/etc.defaults/rc.sysv# ./apparmor.sh stop
Unloading AppArmor profiles
root@test:/usr/syno/etc.defaults/rc.sysv# ./avahi.sh stop
root@test:/usr/syno/etc.defaults/rc.sysv# ./cupsd.sh stop
root@test:/usr/syno/etc.defaults/rc.sysv# ./irqbalance.sh stop
root@test:/usr/syno/etc.defaults/rc.sysv# ./usbip.sh stop
root@test:/# umount -l /dev/mapper/cachedev_0

root@test:/# btrfs filesystem resize -- -40G /volume1
Resize '/volume1' of '-40G'

root@test:/# lvdisplay

--- Logical volume ---
LV Path                /dev/vg1/volume_1
LV Name                volume_1
VG Name                vg1
LV UUID                R4rD6l-bTbD-6P1B-F2jW-jOiN-XZkF-Fb48Lg
LV Write Access        read/write
LV Creation host, time ,
LV Status              available
# open                 1
LV Size                89.00 GiB
Current LE             22784
Segments               1
Allocation             inherit
Read ahead sectors     auto
- currently set to     384
Block device           249:1

root@test:/# lvm lvreduce -L -40G /dev/vg1/volume_1
WARNING: Reducing active and open logical volume to 49.00 GiB
THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce volume_1? [y/n]: y
Size of logical volume vg1/volume_1 changed from 89.00 GiB (22784 extents) to 49.00 GiB (12544 extents).
Logical volume volume_1 successfully resized.

btrfs fi resize max /volume1