Found the error
WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 29440MB of RAM.
dmesg | grep -i memory
[ 0.000000] WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 29440MB of RAM.
[ 0.000000] WARNING: CPU: 0 PID: 0 at arch/x86/kernel/cpu/mtrr/cleanup.c:972 mtrr_trim_uncached_memory+0x458/0x47f()
[ 0.000000] [<ffffffff8190368c>] mtrr_trim_uncached_memory+0x458/0x47f
[ 0.000000] Base memory trampoline at [ffff880000096000] 96000 size 28672
[ 0.000000] Early memory node ranges
[ 0.000000] PM: Registered nosave memory: [mem 0x00000000-0x00000fff]
[ 0.000000] PM: Registered nosave memory: [mem 0x0009e000-0x0009ffff]
[ 0.000000] PM: Registered nosave memory: [mem 0x000a0000-0x000effff]
[ 0.000000] PM: Registered nosave memory: [mem 0x000f0000-0x000fffff]
[ 0.000000] Memory: 2756920K/2881076K available (5548K kernel code, 880K rwdata, 1780K rodata, 924K init, 1568K bss, 124156K reserved, 0K cma-reserved)
Fixed it by adding disable_mtrr_trim to the kernel options. Now I can utilize all available memory. This is probably a BIOS problem with my old GA-78LMT-USB3 FA Motherboard. There are no bios updates available.
free -mh
total used free shared buff/cache available
Mem: 30Gi 1.0Gi 27Gi 139Mi 2.0Gi 29Gi
Swap: 20Gi 0B 20Gi
cat /proc/cmdline
BOOT_IMAGE=/zImage withefi syno_hw_version=DS3622xs+ console=ttyS0,115200n8 disable_mtrr_trim netif_num=1 earlycon=uart8250,io,0x3f8,115200n8 mac1=001132F55B09 sn=20C0SQRMHS98G HddHotplug=0 DiskIdxMap=00 syno_hdd_detect=0 vender_format_version=2 syno_hdd_powerup_seq=0 root=/dev/md0 SataPortMap=4
