libvirt
version: 3.4.0
architecture: x86_64 ubuntu16.04-server
hypervisor: kvm,qemu

when I want make a memery snapshot for a vm ,and I call virsh save ,but tell me this error:
error: Failed to save domain 177 to /datapool/mm.img
error: operation failed: domain save job: unexpectedly failed

xml configure:
<domain type='kvm' id='177'>
<name>virt23</name>
<uuid>e50c1d59-c4a3-4413-b6c9-20e90cced343</uuid>
<metadata/>
<memory unit='KiB'>10485760</memory>
<currentMemory unit='KiB'>10485760</currentMemory>
<vcpu placement='static'>1</vcpu>
<resource>
    <partition>/machine</partition>
</resource>
<os>
    <type arch='x86_64' machine='pc-i440fx-2.5'>hvm</type>
    <boot dev='hd'/>
</os>
<clock offset='utc'/>
<on_poweroff>destroy</on_poweroff>
<on_reboot>restart</on_reboot>
<on_crash>destroy</on_crash>
<devices>
    <emulator>/usr/bin/kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/datapool/zhuohf/virt23.qcow2'/>
      <backingStore/>
      <target dev='vdc' bus='virtio'/>
      <alias name='virtio-disk2'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu' type='raw'/>
<source file='/datapool/zhuohf/ubuntu-16.04.2-desktop-amd64.iso'/>      [40/38156]
      <backingStore/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <alias name='ide0-1-0'/>
      <address type='drive' controller='0' bus='1' target='0' unit='0'/>
    </disk>
    <controller type='usb' index='0' model='piix3-uhci'>
      <alias name='usb'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
    </controller>
    <controller type='pci' index='0' model='pci-root'>
      <alias name='pci.0'/>
    </controller>
    <controller type='ide' index='0'>
      <alias name='ide'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <interface type='network'>
      <mac address='52:54:00:e0:31:68'/>
      <source network='default' bridge='virbr0'/>
      <target dev='vnet11'/>
      <model type='rtl8139'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>
    </interface>
    <input type='mouse' bus='ps2'>
      <alias name='input0'/>
    </input>
    <input type='keyboard' bus='ps2'>
      <alias name='input1'/>
</input>
    <graphics type='vnc' port='5909' autoport='yes' listen='0.0.0.0'>
      <listen type='address' address='0.0.0.0'/>
    </graphics>
    <video>
      <model type='cirrus' vram='16384' heads='1' primary='yes'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
    <memballoon model='virtio'>
      <stats period='1'/>
      <alias name='balloon0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </memballoon>
</devices>
<seclabel type='none' model='none'/>
<seclabel type='dynamic' model='dac' relabel='yes'>
    <label>+0:+0</label>
    <imagelabel>+0:+0</imagelabel>
</seclabel>
</domain>

before I encounter the same error,that time is my disk not free space，but this time I can't find error reason.

Thanks