<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def cli(env, sortby, columns, datacenter, username, storage_type):
 """List block storage."""

    block_manager = SoftLayer.BlockStorageManager(env.client)
    block_volumes = block_manager.list_block_volumes(datacenter=datacenter,
                                                     username=username,
                                                     storage_type=storage_type,
                                                     mask=columns.mask())

    table = formatting.Table(columns.columns)
    table.sortby = sortby

    for block_volume in block_volumes:
        table.add_row([value or formatting.blank()
                       for value in columns.row(block_volume)])

    env.fout(table)