emerge dev-db/xtrabackup-bin runs fine, but : ~ # xtrabackup xtrabackup: error while loading shared libraries: libnuma.so.1: cannot open shared object file: No such file or directory
I can confirm the problem and it still exists with dev-db/xtrabackup-bin-2.4.9 Manually installed sys-process/numactl to fix it.
dev-db/xtrabackup-bin-2.4.9 # ldd /usr/bin/xtrabackup linux-vdso.so.1 (0x000003948075b000) libpthread.so.0 => /lib64/libpthread.so.0 (0x0000039480318000) libaio.so.1 => /lib64/libaio.so.1 (0x0000039480115000) libnuma.so.1 => not found libgcrypt.so.11 => /usr/lib64/libgcrypt.so.11 (0x000003947fe87000) libgpg-error.so.0 => /usr/lib64/libgpg-error.so.0 (0x000003947fc6e000) librt.so.1 => /lib64/librt.so.1 (0x000003947fa65000) libcrypt.so.1 => /lib64/libcrypt.so.1 (0x000003947f82d000) libdl.so.2 => /lib64/libdl.so.2 (0x000003947f629000) libz.so.1 => /lib64/libz.so.1 (0x000003947f411000) libstdc++.so.6 => /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/libstdc++.so.6 (0x000003947efe2000) libm.so.6 => /lib64/libm.so.6 (0x000003947ecc9000) libgcc_s.so.1 => /usr/lib/gcc/x86_64-pc-linux-gnu/6.4.0/libgcc_s.so.1 (0x000003947eab3000) libc.so.6 => /lib64/libc.so.6 (0x000003947e6f7000) /lib64/ld-linux-x86-64.so.2 (0x0000039480539000)
it seems this was dropped some time ago