> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/4.3-bsd-unix-cao-zuo-xi-tong-she-ji-yu-shi-xian/di-san-bu-fen-io-xi-tong/8.3.-kuai-she-bei.md).

# 8.3 块设备

块设备的设备驱动由 `bdevsw` 表中的一个表项描述。块设备最常见的是磁盘机和磁带机。每个 `bdevsw` 结构含以下入口点：

`open`：为 I/O 操作做准备而打开设备。对块特殊设备文件的每次 `open` 系统调用，或在内部为挂载文件系统用 `mount` 系统调用准备设备时，调用设备的 open 入口点。`open()` 例程通常校验关联介质的完好性，例如核实设备在自动配置阶段被识别过、磁带机和磁盘机有介质在线。

`strategy`：发起读或写操作并立即返回。对设备上文件系统的 I/O 请求由系统翻译成对块 I/O 例程 `bread()` 和 `bwrite()` 的调用；这些块 I/O 例程再调用设备的 strategy 例程读写缓存中没有的数据。对 strategy 例程的每次调用给出一个指向 `buf` 结构的指针，其中含 I/O 请求的参数。若请求是同步的，调用者必须（在 `buf` 结构的地址上）入睡等 I/O 完成。

`close`：关闭设备。最后一个关心该设备的客户终止后调用 `close()` 例程——这一语义由更高层的 I/O 设施定义。磁盘设备关闭时无事可做，用空的 `close()` 例程。只支持单客户访问的设备必须把自身重新标记为可用。关闭曾以写方式打开的磁带机，通常导致往磁带写文件结束标记并回卷磁带。

`dump`：把全部物理内存写到设备。dump 入口点用于把内存内容保存到辅存。系统检测到不可恢复错误即将 **崩溃** 时自动转储；转储用于对导致系统崩溃的问题做事后分析。dump 例程在处理器最高优先级下被调用，设备驱动必须轮询设备状态而不能等中断。所有磁盘设备都应支持这一入口点，部分磁带设备也支持。

`psize`：返回磁盘驱动器分区的大小。驱动收到一个逻辑部件号，应返回该部件——通常是磁盘驱动器分区——以 DEV\_BSIZE 块计的大小。这一入口点在自举过程中用来计算崩溃转储的存放位置、确定交换设备的大小。


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