Specifies the largest unit of transfer in bytes to be used between sql server and the backup media . the possible values are multiples of 65536 bytes 64 kb ranging up to 4194304 bytes 4 mb 只用于磁带设备，以便提高对已加载的磁带执行多个备份操作时的性能，指定sql server在备份操作后让磁带一直处于打开状态。
