def remove_snap(pool_name, pool_device, share_name, snap_name):
    full_name = ('%s/%s' % (share_name, snap_name))
    if (is_share_mounted(full_name)):
        umount_root('%s%s' % (DEFAULT_MNT_DIR, full_name))
    root_pool_mnt = mount_root(pool_name, pool_device)
    subvol_mnt_pt = ('%s/%s_%s' % (root_pool_mnt, share_name, snap_name))
    return run_command([BTRFS, 'subvolume', 'delete', subvol_mnt_pt])

def remove_share(pool, share_name, pqgroup, force=False):
    """
    umount share if its mounted.
    mount root pool
    btrfs subvolume delete root_mnt/vol_name
    umount root pool
    """
    if (is_share_mounted(share_name)):
        mnt_pt = ('%s%s' % (DEFAULT_MNT_DIR, share_name))
        umount_root(mnt_pt)
    root_pool_mnt = mount_root(pool)
    subvol_mnt_pt = root_pool_mnt + '/' + share_name
    if (not is_subvol(subvol_mnt_pt)):
        return
    if (force):
        o, e, rc = run_command([BTRFS, 'subvolume', 'list', '-o', subvol_mnt_pt])
        for l in o:
            if (re.match('ID ', l) is not None):
                subvol = root_pool_mnt + '/' + l.split()[-1]
                run_command([BTRFS, 'subvolume', 'delete', subvol], log=True)
    qgroup = ('0/%s' % share_id(pool, share_name))
    delete_cmd = [BTRFS, 'subvolume', 'delete', subvol_mnt_pt]
    run_command(delete_cmd, log=True)
    qgroup_destroy(qgroup, root_pool_mnt)
    return qgroup_destroy(pqgroup, root_pool_mnt)

    def post(self, request, command):
        if (command == 'bootstrap'):

            self._refresh_pool_state()
            for p in Pool.objects.all():
                import_shares(p, request)

            for share in Share.objects.all():
                try:
                    if (share.pqgroup == settings.MODEL_DEFS['pqgroup']):
                        share.pqgroup = qgroup_create(share.pool)
                        share.save()
                    if (not is_share_mounted(share.name)):
                        mnt_pt = ('%s%s' % (settings.MNT_PT, share.name))
                        mount_share(share, mnt_pt)
                except Exception, e:
                    e_msg = ('Exception while mounting a share(%s) during '
                             'bootstrap: %s' % (share.name, e.__str__()))
                    logger.error(e_msg)
                    logger.exception(e)

                try:
                    import_snapshots(share)
                except Exception, e:
                    e_msg = ('Exception while importing Snapshots of '
                             'Share(%s): %s' % (share.name, e.__str__()))
                    logger.error(e_msg)
                    logger.exception(e)

def mount_root(pool_name, device):
    root_pool_mnt = DEFAULT_MNT_DIR + pool_name
    if (is_share_mounted(pool_name)):
        return root_pool_mnt
    create_tmp_dir(root_pool_mnt)
    mnt_cmd = [MOUNT, '-t', 'btrfs', device, root_pool_mnt]
    run_command(mnt_cmd)
    return root_pool_mnt

    def run(self):
        #0. mount destination share
        dest_mnt_pt = ('%s/%s' % (settings.MNT_PT, self.dest_share))
        msg = ('Destination share(%s) not mounted' % self.dest_share)
        with self._update_trail_and_quit(msg):
            if (not is_share_mounted(self.dest_share)):
                mount_share(self.dest_share, self.dest_pool, dest_mnt_pt)
            if (not is_share_mounted(self.dest_share)):
                raise Exception(msg)

        #1. mount source share
        msg = ('Failed to mount source(%s:%s)' % (self.source_ip,
                                                  self.source_path))
        with self._update_trail_and_quit(msg):
            mount_source(self.source_ip, self.source_path)

        #2. create a snapshot
        snap_name = ('snap-%d' % self.tid)
        msg = ('Failed to create snapshot(%s) for share(%s)' %
               (snap_name, self.dest_share))
        data = {'status': 'snapshot created',}
        with self._update_trail_and_quit(msg, data=data):
            create_snapshot(self.dest_share, snap_name, logger)

        #3. rsync
        src_mnt = ('/mnt/backup/%s_%s' % (self.source_ip, self.source_path))
        cmd = [RSYNC, '-az', src_mnt, dest_mnt_pt]
        msg = ('Unable to start sync')
        data = {'status': 'sync started',}
        with self._update_trail_and_quit(msg, data=data):
            rp = subprocess.Popen(cmd, shell=False, stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE)
        while True:
            if (os.getppid() != self.ppid):
                msg = ('Backup plugin scheduler exited. Aborting...')
                with self._update_trail_and_quit(msg):
                    rp.terminate()

            if (rp.poll() is not None):
                msg = ('sync finished. But failed to update trail status')
                data = {'status': 'succeeded',}
                with self._update_trail_and_quit(msg, data):
                    logger.debug('sync finished')
                break
            time.sleep(1)

def remove_snap_old(pool, pool_device, share_name, snap_name):
    full_name = "%s/%s" % (share_name, snap_name)
    if is_share_mounted(full_name):
        umount_root("%s%s" % (DEFAULT_MNT_DIR, full_name))
    root_pool_mnt = mount_root(pool, pool_device)
    subvol_mnt_pt = "%s/%s_%s" % (root_pool_mnt, share_name, snap_name)
    if is_subvol(subvol_mnt_pt):
        return run_command([BTRFS, "subvolume", "delete", subvol_mnt_pt])
    return True

def mount_root(pool):
    """
    Mounts a given pool at the default mount root (usually /mnt2/) using the
    pool.name as the final path entry. Ie pool.name = test-pool will be mounted
    at /mnt2/test-pool. Any mount options held in pool.mnt_options will be added
    to the mount command via the -o option as will a compress=pool.compression
    entry.
    N.B. Initially the mount target is defined by /dev/disk/by-label/pool.name,
    if this fails then an attempt to mount by each member of
    /dev/disk/by-id/pool.disk_set.all() but only if there are any members.
    If this second method also fails then an exception is raised, currently all
    but the last failed mount by device name is logged. If no disk members were
    reported by pool.disk_set.count() a separate Exception is raised.
    :param pool: pool object
    :return: either the relevant mount point or an Exception which either
    indicates 'no disks in pool' or 'Unknown Reason'
    """
    root_pool_mnt = DEFAULT_MNT_DIR + pool.name
    if (is_share_mounted(pool.name)):
        return root_pool_mnt
    # Creates a directory to act as the mount point.
    create_tmp_dir(root_pool_mnt)
    mnt_device = '/dev/disk/by-label/%s' % pool.name
    mnt_cmd = [MOUNT, mnt_device, root_pool_mnt, ]
    mnt_options = ''
    if (pool.mnt_options is not None):
        mnt_options = pool.mnt_options
    if (pool.compression is not None):
        if (re.search('compress', mnt_options) is None):
            mnt_options = ('%s,compress=%s' % (mnt_options, pool.compression))
    if (os.path.exists(mnt_device)):
        if (len(mnt_options) > 0):
            mnt_cmd.extend(['-o', mnt_options])
        run_command(mnt_cmd)
        return root_pool_mnt
    # If we cannot mount by-label, let's try mounting by device; one by one
    # until we get our first success.
    if (pool.disk_set.count() < 1):
        raise Exception('Cannot mount Pool(%s) as it has no disks in it.' % pool.name)
    last_device = pool.disk_set.last()
    for device in pool.disk_set.all():
        mnt_device = ('/dev/disk/by-id/%s' % device.name)
        if (os.path.exists(mnt_device)):
            mnt_cmd = [MOUNT, mnt_device, root_pool_mnt, ]
            if (len(mnt_options) > 0):
                mnt_cmd.extend(['-o', mnt_options])
            try:
                run_command(mnt_cmd)
                return root_pool_mnt
            except Exception, e:
                if (device.name == last_device.name):
                    # exhausted mounting using all devices in the pool
                    raise e
                logger.error('Error mounting: %s. Will try using another device.' % mnt_cmd)
                logger.exception(e)

def rollback_snap(snap_name, sname, subvol_name, pool_name, pool_device):
    """
    1. umount the share
    2. mount the snap as the share
    3. remove the share
    """
    mnt_pt = ('%s%s' % (DEFAULT_MNT_DIR, sname))
    if (is_share_mounted(sname)):
        umount_root(mnt_pt)
    mount_share(snap_name, pool_device, mnt_pt)
    remove_share(pool_name, pool_device, subvol_name)

    def post(self, request):
        with self._handle_exception(request):
            pool_name = request.data.get('pool', None)
            try:
                pool = Pool.objects.get(name=pool_name)
            except:
                e_msg = ('Pool(%s) does not exist.' % pool_name)
                handle_exception(Exception(e_msg), request)
            compression = self._validate_compression(request)
            size = self._validate_share_size(request, pool)
            sname = request.data.get('sname', None)
            if ((sname is None or
                 re.match('%s$' % settings.SHARE_REGEX, sname) is None)):
                e_msg = ('Share name must start with a alphanumeric(a-z0-9) '
                         'character and can be followed by any of the '
                         'following characters: letter(a-z), digits(0-9), '
                         'hyphen(-), underscore(_) or a period(.).')
                handle_exception(Exception(e_msg), request)

            if (Share.objects.filter(name=sname).exists()):
                e_msg = ('Share(%s) already exists. Choose a different name' % sname)
                handle_exception(Exception(e_msg), request)

            if (Pool.objects.filter(name=sname).exists()):
                e_msg = ('A Pool with this name(%s) exists. Share and Pool names '
                         'must be distinct. Choose a different name' % sname)
                handle_exception(Exception(e_msg), request)
            disk = Disk.objects.filter(pool=pool)[0]
            replica = False
            if ('replica' in request.data):
                replica = request.data['replica']
                if (type(replica) != bool):
                    e_msg = ('replica must be a boolean, not %s' %
                             type(replica))
                    handle_exception(Exception(e_msg), request)
            add_share(pool, disk.name, sname)
            qid = qgroup_id(pool, disk.name, sname)
            update_quota(pool, disk.name, qid, size * 1024)
            s = Share(pool=pool, qgroup=qgroup_id, name=sname, size=size,
                      subvol_name=sname, replica=replica,
                      compression_algo=compression)
            s.save()
            mnt_pt = '%s%s' % (settings.MNT_PT, sname)
            if (not is_share_mounted(sname)):
                disk = Disk.objects.filter(pool=pool)[0].name
                mount_share(s, disk, mnt_pt)
            if (compression != 'no'):
                set_property(mnt_pt, 'compression', compression)
            return Response(ShareSerializer(s).data)

def remove_share(pool_name, pool_device, share_name):
    """
    umount share if its mounted.
    mount root pool
    btrfs subvolume delete root_mnt/vol_name
    umount root pool
    """
    if (is_share_mounted(share_name)):
        mnt_pt = ('%s%s' % (DEFAULT_MNT_DIR, share_name))
        umount_root(mnt_pt)
    pool_device = '/dev/' + pool_device
    root_pool_mnt = mount_root(pool_name, pool_device)
    subvol_mnt_pt = root_pool_mnt + '/' + share_name
    delete_cmd = [BTRFS, 'subvolume', 'delete', subvol_mnt_pt]
    run_command(delete_cmd)

def remove_share(pool, pool_device, share_name):
    """
    umount share if its mounted.
    mount root pool
    btrfs subvolume delete root_mnt/vol_name
    umount root pool
    """
    if is_share_mounted(share_name):
        mnt_pt = "%s%s" % (DEFAULT_MNT_DIR, share_name)
        umount_root(mnt_pt)
    pool_device = "/dev/" + pool_device
    root_pool_mnt = mount_root(pool, pool_device)
    subvol_mnt_pt = root_pool_mnt + "/" + share_name
    if not is_subvol(subvol_mnt_pt):
        return
    delete_cmd = [BTRFS, "subvolume", "delete", subvol_mnt_pt]
    run_command(delete_cmd)

def mount_root(pool, device):
    root_pool_mnt = DEFAULT_MNT_DIR + pool.name
    if is_share_mounted(pool.name):
        return root_pool_mnt
    create_tmp_dir(root_pool_mnt)
    mnt_device = "/dev/disk/by-label/%s" % pool.name
    if not os.path.exists(mnt_device):
        mnt_device = device
    mnt_cmd = [MOUNT, mnt_device, root_pool_mnt]
    mnt_options = ""
    if pool.mnt_options is not None:
        mnt_options = pool.mnt_options
    if pool.compression is not None:
        if re.search("compress", mnt_options) is None:
            mnt_options = "%s,compress=%s" % (mnt_options, pool.compression)
    if len(mnt_options) > 0:
        mnt_cmd.extend(["-o", mnt_options])
    run_command(mnt_cmd)
    return root_pool_mnt

def remove_share(pool, share_name, pqgroup):
    """
    umount share if its mounted.
    mount root pool
    btrfs subvolume delete root_mnt/vol_name
    umount root pool
    """
    if (is_share_mounted(share_name)):
        mnt_pt = ('%s%s' % (DEFAULT_MNT_DIR, share_name))
        umount_root(mnt_pt)
    root_pool_mnt = mount_root(pool)
    subvol_mnt_pt = root_pool_mnt + '/' + share_name
    if (not is_subvol(subvol_mnt_pt)):
        return
    qgroup = ('0/%s' % share_id(pool, share_name))
    delete_cmd = [BTRFS, 'subvolume', 'delete', subvol_mnt_pt]
    run_command(delete_cmd, log=True)
    qgroup_destroy(qgroup, root_pool_mnt)
    return qgroup_destroy(pqgroup, root_pool_mnt)

def remove_share(pool, share_name, pqgroup):
    """
    umount share if its mounted.
    mount root pool
    btrfs subvolume delete root_mnt/vol_name
    umount root pool
    """
    if is_share_mounted(share_name):
        mnt_pt = "%s%s" % (DEFAULT_MNT_DIR, share_name)
        umount_root(mnt_pt)
    root_pool_mnt = mount_root(pool)
    subvol_mnt_pt = root_pool_mnt + "/" + share_name
    if not is_subvol(subvol_mnt_pt):
        return
    qgroup = "0/%s" % share_id(pool, share_name)
    delete_cmd = [BTRFS, "subvolume", "delete", subvol_mnt_pt]
    run_command(delete_cmd, log=True)
    qgroup_destroy(qgroup, root_pool_mnt)
    return qgroup_destroy(pqgroup, root_pool_mnt)

def shares_usage(pool_name, pool_device, share_map, snap_map):
    #don't mount the pool if at least one share in the map is mounted.
    usage_map = {}
    mnt_pt = None
    for s in share_map.keys():
        if (is_share_mounted(share_map[s])):
            mnt_pt = ('%s%s' % (DEFAULT_MNT_DIR, share_map[s]))
            break
    if (mnt_pt is None):
        mnt_pt = mount_root(pool_name, '/dev/' + pool_device)
    cmd = [BTRFS, 'qgroup', 'show', mnt_pt]
    out, err, rc = run_command(cmd)
    combined_map = dict(share_map, **snap_map)
    for line in out:
        fields = line.split()
        if (len(fields) > 0 and fields[0] in combined_map):
            r_usage = int(fields[-2]) / 1024 # referenced usage in KB
            e_usage = int(fields[-1]) / 1024 # exclusive usage in KB
            usage_map[combined_map[fields[0]]] = (r_usage, e_usage)
    return usage_map

def shares_usage(pool, share_map, snap_map):
    # don't mount the pool if at least one share in the map is mounted.
    usage_map = {}
    mnt_pt = None
    for s in share_map.keys():
        if (is_share_mounted(share_map[s])):
            mnt_pt = ('%s%s' % (DEFAULT_MNT_DIR, share_map[s]))
            break
    if (mnt_pt is None):
        mnt_pt = mount_root(pool)
    cmd = [BTRFS, 'qgroup', 'show', mnt_pt]
    out, err, rc = run_command(cmd, log=True)
    combined_map = dict(share_map, **snap_map)
    for line in out:
        fields = line.split()
        if (len(fields) > 0 and fields[0] in combined_map):
            r_usage = convert_to_KiB(fields[-2])
            e_usage = convert_to_KiB(fields[-1])
            usage_map[combined_map[fields[0]]] = (r_usage, e_usage)
    return usage_map

def rollback_snap(snap_name, sname, subvol_name, pool, pool_device):
    """
    1. validate destination snapshot and umount the share
    2. remove the share
    3. move the snapshot to share location and mount it.
    """
    mnt_pt = "%s%s" % (DEFAULT_MNT_DIR, sname)
    snap_fp = "%s/%s/.snapshots/%s/%s" % (DEFAULT_MNT_DIR, pool.name, sname, snap_name)
    if not is_subvol(snap_fp):
        raise Exception("Snapshot(%s) does not exist. Rollback is not " "possible" % snap_fp)
    dpath = "/dev/%s" % pool_device
    mount_root(pool, dpath)
    if is_share_mounted(sname):
        umount_root(mnt_pt)
    remove_share(pool, pool_device, subvol_name)
    shutil.move(snap_fp, "%s/%s/%s" % (DEFAULT_MNT_DIR, pool.name, sname))
    create_tmp_dir(mnt_pt)
    subvol_str = "subvol=%s" % sname
    mnt_cmd = [MOUNT, "-t", "btrfs", "-o", subvol_str, dpath, mnt_pt]
    run_command(mnt_cmd)

def mount_root(pool, device):
    root_pool_mnt = DEFAULT_MNT_DIR + pool.name
    if (is_share_mounted(pool.name)):
        return root_pool_mnt
    create_tmp_dir(root_pool_mnt)
    mnt_device = '/dev/disk/by-label/%s' % pool.name
    if (not os.path.exists(mnt_device)):
        mnt_device = device
        if (re.match('/dev/', device) is None):
            mnt_device = ('/dev/%s' % mnt_device)
    mnt_cmd = [MOUNT, mnt_device, root_pool_mnt, ]
    mnt_options = ''
    if (pool.mnt_options is not None):
        mnt_options = pool.mnt_options
    if (pool.compression is not None):
        if (re.search('compress', mnt_options) is None):
            mnt_options = ('%s,compress=%s' % (mnt_options, pool.compression))
    if (len(mnt_options) > 0):
        mnt_cmd.extend(['-o', mnt_options])
    run_command(mnt_cmd)
    return root_pool_mnt

def mount_root(pool):
	root_pool_mnt = DEFAULT_MNT_DIR + pool["name"]
	if (is_share_mounted(pool["name"])):
		return root_pool_mnt
	create_tmp_dir(root_pool_mnt)
	mnt_device = '/dev/disk/by-label/%s' % pool["name"]
	mnt_cmd = [MOUNT, mnt_device, root_pool_mnt, ]
	mnt_options = ''
	if (pool["mnt_options"] is not None):
		mnt_options = pool["mnt_options"]
	if (pool["compression"] is not None):
		if (re.search('compress', mnt_options) is None):
			mnt_options = ('%s,compress=%s' % (mnt_options, pool["compression"]))
	if (os.path.exists(mnt_device)):
		if (len(mnt_options) > 0):
			mnt_cmd.extend(['-o', mnt_options])
		run_command(mnt_cmd)
		return root_pool_mnt

	# If we cannot mount by-label, let's try mounting by device one by one
	# until we get our first success.
	# ==============================================================================#
	if (len(pool["disks"]) < 1):
		raise Exception('Cannot mount Pool(%s) as it has no disks in it.' % pool["name"])
	last_device = pool["disks"][-1]
	for device in pool["disks"]:
		mnt_device = ('/dev/%s' % device["name"])
		if (os.path.exists(mnt_device)):
			mnt_cmd = [MOUNT, mnt_device, root_pool_mnt, ]
			if (len(mnt_options) > 0):
				mnt_cmd.extend(['-o', mnt_options])
			try:
				run_command(mnt_cmd)
				return root_pool_mnt
			except Exception, e:
				if (device["name"] == last_device["name"]):
					# exhausted mounting using all devices in the pool
					raise e
				logger.error('Error mouting: %s. Will try using another device.' % mnt_cmd)
				logger.exception(e)

    def post(self, request, command):
        if (command == 'bootstrap'):
            for pool in Pool.objects.all():
                try:
                    mount_root(pool)
                except Exception, e:
                    e_msg = ('Exception while mounting a pool(%s) during '
                             'bootstrap: %s' % (pool.name, e.__str__()))
                    logger.error(e_msg)

            for share in Share.objects.all():
                try:
                    if (share.pqgroup == settings.MODEL_DEFS['pqgroup']):
                        share.pqgroup = qgroup_create(share.pool)
                        share.save()
                    if (not is_share_mounted(share.name)):
                        mnt_pt = ('%s%s' % (settings.MNT_PT, share.name))
                        mount_share(share, mnt_pt)
                except Exception, e:
                    e_msg = ('Exception while mounting a share(%s) during '
                             'bootstrap: %s' % (share.name, e.__str__()))
                    logger.error(e_msg)