C.3. Glocks

要了解 GFS2，最重要的概念，也是与其他文件系统不同的概念就是 glocks。就源代码而言，glock 是一个数据结构，可将 DLM 和缓存带入一个单一静态机器。每个 glock 都与单一 DLM 锁定有 1:1 对应关系，并为那个锁定状态提供缓存，这样来自文件系统单一节点的竞争操作就不会重复调用 DLM，因此可帮助避免不必要的网络流量。glocks 有两个大类，即缓冲元数据的一类和不缓冲元数据的一类。内节点 glock 和资源组 glock 都可缓冲元数据，其他 glock 类型则不缓冲元数据。内节点 glock 还包括缓冲元数据以外的数据，并拥有所有 glock 中最复杂的逻辑。

Glocks remain in memory until either they are unlocked (at the request of another node or at the request of the VM) and there are no local users. At that point they are removed from the glock hash table and freed. When a glock is created, the DLM lock is not associated with the glock immediately. The DLM lock becomes associated with the glock upon the first request to the DLM, and if this request is successful then the 'I' (initial) flag will be set on the glock. 表 C.4 “Glock 标签” shows the meanings of the different glock flags. Once the DLM has been associated with the glock, the DLM lock will always remain at least at NL (Null) lock mode until the glock is to be freed. A demotion of the DLM lock from NL to unlocked is always the last operation in the life of a glock.

Each glock can have a number of "holders" associated with it, each of which represents one lock request from the higher layers. System calls relating to GFS2 queue and dequeue holders from the glock to protect the critical section of code.

使用 glock 的机器是基于工作队列（workqueue）。出于性能考虑，更倾向于使用小任务（tasklet），但在当前部署中需要从禁止其使用的上下文中提交 I/O。

表 C.2 “Glock 模式和数据类型” 显示每个 glock 模块下可能缓存的状态是什么，以及缓存的状态是否 dirty。这可适用于内节点和资源组锁，虽然资源组锁中没有数据内容，只有元数据。