MDS States

後設資料伺服器（MDS）在CephFS的正常操作過程中經歷多個狀態。例如，一些狀態指示MDS從MDS的先前例項從故障轉移中恢復。在這裡，我們將記錄所有這些狀態，幷包括狀態圖來視覺化轉換。

State Descriptions

Common states

狀態	說明
up:active	This is the normal operating state of the MDS. It indicates that the MDS and its rank in the file system is available. 這個狀態是正常執行的狀態。 這個表明該mds在rank中是可用的狀態。
up:standby	The MDS is available to takeover for a failed rank (see also :ref:mds-standby). The monitor will automatically assign an MDS in this state to a failed rank once available. 這個狀態是災備狀態，用來接替主掛掉的情況。
up:standby_replay	The MDS is following the journal of another up:active MDS. Should the active MDS fail, having a standby MDS in replay mode is desirable as the MDS is replaying the live journal and will more quickly takeover. A downside to having standby replay MDSs is that they are not available to takeover for any other MDS that fails, only the MDS they follow. 災備守護程式就會持續讀取某個處於 up 狀態的 rank 的後設資料日誌。這樣它就有後設資料的熱快取，在負責這個 rank 的守護程式失效時，可加速故障切換。 一個正常執行的 rank 只能有一個災備重放守護程式（ standby replay daemon ），如果兩個守護程式都設定成了災備重放狀態，那麼其中任意一個會取勝，另一個會變為普通的、非重放災備狀態。 一旦某個守護程式進入災備重放狀態，它就只能為它那個 rank 提供災備。如果有另外一個 rank 失效了，即使沒有災備可用，這個災備重放守護程式也不會去頂替那個失效的。
up:boot	This state is broadcast to the Ceph monitors during startup. This state is never visible as the Monitor immediately assign the MDS to an available rank or commands the MDS to operate as a standby. The state is documented here for completeness. 此狀態在啟動期間被廣播到CEPH監視器。這種狀態是不可見的，因為監視器立即將MDS分配給可用的秩或命令MDS作為備用操作。這裡記錄了完整性的狀態。
up:creating	The MDS is creating a new rank (perhaps rank 0) by constructing some per-rank metadata (like the journal) and entering the MDS cluster.
up:starting	The MDS is restarting a stopped rank. It opens associated per-rank metadata and enters the MDS cluster.
up:stopping	When a rank is stopped, the monitors command an active MDS to enter the up:stopping state. In this state, the MDS accepts no new client connections, migrates all subtrees to other ranks in the file system, flush its metadata journal, and, if the last rank (0), evict all clients and shutdown (see also :ref:cephfs-administration).
up:replay	The MDS taking over a failed rank. This state represents that the MDS is recovering its journal and other metadata. 日誌恢復階段，他將日誌內容讀入記憶體後，在記憶體中進行回放操作。
up:resolve	The MDS enters this state from up:replay if the Ceph file system has multiple ranks (including this one), i.e. it's not a single active MDS cluster. The MDS is resolving any uncommitted inter-MDS operations. All ranks in the file system must be in this state or later for progress to be made, i.e. no rank can be failed/damaged or up:replay. 用於解決跨多個mds出現權威後設資料分歧的場景，對於服務端包括子樹分佈、Anchor表更新等功能，客戶端包括rename、unlink等操作。
up:reconnect	An MDS enters this state from up:replay or up:resolve. This state is to solicit reconnections from clients. Any client which had a session with this rank must reconnect during this time, configurable via mds_reconnect_timeout. 恢復的mds需要與之前的客戶端重新建立連線，並且需要查詢之前客戶端釋出的檔案控制程式碼，重新在mds的快取中建立一致性功能和鎖的狀態。mds不會同步記錄檔案開啟的資訊，原因是需要避免在訪問mds時產生多餘的延遲，並且大多數檔案是以只讀方式開啟。
up:rejoin	The MDS enters this state from up:reconnect. In this state, the MDS is rejoining the MDS cluster cache. In particular, all inter-MDS locks on metadata are reestablished. If there are no known client requests to be replayed, the MDS directly becomes up:active from this state. 把客戶端的inode載入到mds cache
up:clientreplay	The MDS may enter this state from up:rejoin. The MDS is replaying any client requests which were replied to but not yet durable (not journaled). Clients resend these requests during up:reconnect and the requests are replayed once again. The MDS enters up:active after completing replay.
down:failed	No MDS actually holds this state. Instead, it is applied to the rank in the file system
down:damaged	No MDS actually holds this state. Instead, it is applied to the rank in the file system
down:stopped	No MDS actually holds this state. Instead, it is applied to the rank in the file system

主從切換流程：

• handle_mds_map state change up:boot --> up:replay
• handle_mds_map state change up:replay --> up:reconnect
• handle_mds_map state change up:reconnect --> up:rejoin
• handle_mds_map state change up:rejoin --> up:active

State Diagram

This state diagram shows the possible state transitions for the MDS/rank. The legend is as follows:

Color

• 綠色: MDS是活躍的.
• 橙色: MDS處於過渡臨時狀態，試圖變得活躍.
• 紅色: MDS指示一個狀態，該狀態導致被標記為失敗.
• 紫色: MDS和rank為停止.
• 紅色: MDS指示一個狀態，該狀態導致被標記為損壞.

Shape

• 圈：MDS保持這種狀態.
• 六邊形：沒有MDS保持這個狀態.

Lines

• A double-lined shape indicates the rank is "in".

參考：
https://github.com/ceph/ceph/blob/master/doc/cephfs/mds-states.rst