An IO error happen when qemu  snapshot-create

My qemu version is 1.7.1,but when I try to make live snapshot by libvirt,the libvirt sometimes report an error :qemuMonitorJSONCheckError:377 : internal error: unable to execute QEMU command 'transaction': An IO error has occurred.

Here is the command being snpshot create by virsh:
virsh  snapshot-create snapshot-test.vm   snapshot.xml  --no-metadata  --disk-only --reuse-external
the snapshot.xml:
<domainsnapshot>
  <description>test</description>
  <disks>
    <disk name='vda' snapshot="external">
      <source dev='/home/disk/sbd8' file='/home/disk/sdb8'  type="block"/>
    </disk>
  </disks>
</domainsnapshot>


I have read the qemu code about the snapshot create, and I find the qemu when call the function handle_aiocb_rw_linear():
static ssize_t handle_aiocb_rw_linear(RawPosixAIOData *aiocb, char *buf)
{
    ssize_t offset = 0;
    ssize_t len;

    while (offset < aiocb->aio_nbytes) {
        if (aiocb->aio_type & QEMU_AIO_WRITE) {
            len = pwrite(aiocb->aio_fildes,
                         (const char *)buf + offset,
                         aiocb->aio_nbytes - offset,
                         aiocb->aio_offset + offset);
        } else {
            len = pread(aiocb->aio_fildes,
                        buf + offset,
                        aiocb->aio_nbytes - offset,
                        aiocb->aio_offset + offset);
        }
        if (len == -1 && errno == EINTR) {
            continue;
        } else if (len == -1) {
            offset = -errno;
            break;
        } else if (len == 0) {
            break;
        }
        offset += len;
    }

    return offset;
}

The function pwrite happen error,the errono is 1,and the  error describe:"pwrite  failed, Operation not permitted (1, EPERM) because the process does not have the appropriate privileges to use the pwrite system call".
The qemu call stack about is:
external_snapshot_prepare()->bdrv_flush()->...->paio_submit->...->handle_aiocb_rw_linear.

Please ask the libvirt community for help.  This issue is probably related to your libvirt storage setup.

On Tue, Oct 13, 2015 at 11:52:26AM -0000, jiangchi68 wrote:
> My qemu version is 1.7.1,but when I try to make live snapshot by
> libvirt,the libvirt sometimes report an error
> :qemuMonitorJSONCheckError:377 : internal error: unable to execute QEMU
> command 'transaction': An IO error has occurred.
> 
> Here is the command being snpshot create by virsh:
> virsh  snapshot-create snapshot-test.vm   snapshot.xml  --no-metadata  --disk-only --reuse-external
> the snapshot.xml:
> <domainsnapshot>
>   <description>test</description>
>   <disks>
>     <disk name='vda' snapshot="external">
>       <source dev='/home/disk/sbd8' file='/home/disk/sdb8'  type="block"/>
>     </disk>
>   </disks>
> </domainsnapshot>

Please send your question to <email address hidden>.

It could be related to /home/disk file permissions.