我发现创建快照时出现问题，如下：
[root@centos7 ~]# vgs centos
VG #PV #LV #SN Attr VSize VFree
centos 3 2 0 wz--n- 9,50g 1,99g
[root@centos7 ~]# LANG=C lvcreate --size 300M --snapshot --name alfa /dev/mapper/centos-root device-mapper: reload ioctl on failed: Invalid argument
Aborting. Failed to activate snapshot exception store.
[root@centos7 ~]# dmesg | tail
[ 21.881931] ip6_tables: (C) 2000-2006 Netfilter Core Team
[ 22.144349] Ebtables v2.0 registered
[ 22.205835] Bridge firewalling registered
[ 23.694962] IPv6: ADDRCONF(NETDEV_UP): vhgnet0: link is not ready
[ 23.695831] e1000: vhgnet0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
[ 23.696502] IPv6: ADDRCONF(NETDEV_CHANGE): vhgnet0: link becomes ready
[ 41.871876] device-mapper: table: 253:6: linear: dm-linear: Device lookup failed
[ 41.872657] device-mapper: ioctl: error adding target to table
[ 737.566773] device-mapper: table: 253:7: linear: dm-linear: Device lookup failed
[ 737.567120] device-mapper: ioctl: error adding target to table
这是怎么回事？