是一個加快實例恢復的參數，我們可以根據服務級別來定義一個合理的、可接受的值，該值的單位爲秒。比如設定爲60s，即1分鐘。
假定該值處於合理的情況之下，則一旦實例崩潰，在60s以內實例應當能夠被恢復。合理即是該值不能太大，也不能太小。太大則實例恢復所需的時間較長，太小則導致大量數據的及時寫入，增加了系統的I/O。
影響實例恢復時間長短的主要因素即是從最近檢查點位置到聯機重做日誌尾部之間的距離。距離越長則所需要的cache recovery 和undo、redo的時間越長。所以如何有效的縮短最近檢查點位置與聯機重做日誌尾部之間的距離，這正是FAST_START_MTTR_TARGET的目的。
關於檢查點的觸發條件有很多，比如日誌切換、數據庫shutdown、開始結束備份表空間等。檢查點的分類也很多，比如完全檢查點、部分檢查點、增量檢查點等。
FAST_START_MTTR_TARGET的值實際上也是觸發檢查點的一個觸發條件。當內存中產生的dirty buffer所需的恢復時間(estimated_mttr)
如果到達FAST_START_MTTR_TARGET的指定時間，則檢查點進程被觸發。檢查點進程一旦被觸發，將通知DBWn進程將按檢查點隊列順序將髒數據寫入到數據文件，從而縮短了最後檢查點位置與聯機重做日誌間的距離，減少了實例恢復所需的時間。
關於實例的恢復以及檢查的具體分類描述等，請參考：
Oracle 實例恢復
Oracle實例和Oracle數據庫(Oracle體系結構)
二、FAST_START_MTTR_TARGET = 0的質疑
很多文章描述了FAST_START_MTTR_TARGET = 0，即爲未設置，表示啓用自動檢查點功能，下面是來自Oracle的官方文檔中的一段，
原文的鏈接爲：Fast-Start Fault Recovery
Fast-start checkpointing refers to the periodic writes by the database writer (DBWn) processes for the purpose of writing changed data blocks from the Oracle buffer cache to disk and advancing the thread-checkpoint. Setting the database parameter FAST_START_MTTR_TARGET to a value greater than zero enables the fast-start checkpointing feature. Fast-start checkpointing should always be enabled for the following reasons:
It reduces the time required for cache recovery, and makes instance recovery time-bounded and predictable. This is accomplished by limiting the number of dirty buffers (data blocks which have changes in memory that still need to be written to disk) and the number of redo records (changes in the database) generated between the most recent redo record and the last checkpoint.
Fast-Start checkpointing eliminates bulk writes and corresponding I/O spikes that occur traditionally with interval- based checkpoints, providing a smoother, more consistent I/O pattern that is more predictable and easier to manage. If the system is not already near or at its maximum I/O capacity, fast-start checkpointing will have a negligible impact on performance. Although fast-start checkpointing results in increased write activity, there is little reduction in database throughout, provided the system has sufficient I/O capacity.
從第一段中粗體標記的描述來看，當設定一個大於0的值給FAST_START_MTTR_TARGET，則自動調整檢查點功能別啓用。即fast-start checkpointing，更準確的說應該是快速啓動檢查點功能
再看下面的這段描述，這段來自Oracle 10g OCP workshop I 14-17 英文版教程(Edition 3.1 December 2008)
Explicit setting of the FAST_START_MTTR_TARGET parameter to 0 disables automatic checkpoint tuning.Explicit setting of the FAST_START_MTTR_TARGET parameter to a value other than 0 also enables the Redo Log Advisor.
從上面的描述可以看出，如果將FAST_START_MTTR_TARGET設置爲將關閉檢查點自動調整功能。
三、設定FAST_START_MTTR_TARGET
根據實際需要來設定FAST_START_MTTR_TARGET的值，這個值的設定需要考慮到可接受的實例的恢復時間、可承受的I/O吞吐量等等。
假定我們將該值設定爲
SQL> alter system set fast_start_mttr_target = 30 ;
在事務頻繁的時間段來考察視圖v$instacne_recovery提供的值
SQL> desc v$instance_recovery; --查看v$instance_recovery視圖的結構
Name Null? Type
----------------------------------------- -------- ----------------------------
RECOVERY_ESTIMATED_IOS NUMBER
ACTUAL_REDO_BLKS NUMBER
TARGET_REDO_BLKS NUMBER
LOG_FILE_SIZE_REDO_BLKS NUMBER
LOG_CHKPT_TIMEOUT_REDO_BLKS NUMBER
LOG_CHKPT_INTERVAL_REDO_BLKS NUMBER
FAST_START_IO_TARGET_REDO_BLKS NUMBER
TARGET_MTTR NUMBER
ESTIMATED_MTTR NUMBER
CKPT_BLOCK_WRITES NUMBER
OPTIMAL_LOGFILE_SIZE NUMBER
ESTD_CLUSTER_AVAILABLE_TIME NUMBER
WRITES_MTTR NUMBER
WRITES_LOGFILE_SIZE NUMBER
WRITES_LOG_CHECKPOINT_SETTINGS NUMBER
WRITES_OTHER_SETTINGS NUMBER
WRITES_AUTOTUNE NUMBER
WRITES_FULL_THREAD_CKPT NUMBER
兩個字段：
TARGET_MTTR -->參照fast_start_mttr_target參數中設定的值計算出來的一個值
ESTIMATED_MTTR -->系統根據dirty buffer 中計算出來的值
可能出現的情況
1.TARGET_MTTR > ESTIMATED_MTTR --大量的事務將導致這種情況的出現
2.TARGET_MTTR < ESTIMATED_MTTR --數據庫剛剛啓動時，幾乎沒有事務時會出現這種情況
SQL> select recovery_estimated_ios,actual_redo_blks ,target_redo_blks ,
2 target_mttr,estimated_mttr
3 from v$instance_recovery;
RECOVERY_ESTIMATED_IOS ACTUAL_REDO_BLKS TARGET_REDO_BLKS TARGET_MTTR ESTIMATED_MTTR
---------------------- ---------------- ---------------- ----------- --------------
55 147 707 33 27
可以在負載的情況下根據TARGET_MTTR來值通過v$mttr_target_advice調整該參數
四、啓用MTTR Advisory
需要設置兩個參數
STATISTICS_LEVEL -->置爲typical 或者all
FAST_START_MTTR_TARGET -->置爲非零值
SQL> show parameter mttr; --目標mttr_time設定爲30 s
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
fast_start_mttr_target integer 30
SQL> select target_mttr,estimated_mttr from v$instance_recovery; --系統計算出來的mttr爲33
TARGET_MTTR ESTIMATED_MTTR
----------- --------------
33 27
SQL> select mttr_target_for_estimate tar_est,dirty_limit,estd_cache_writes est_c_w,
2 estd_cache_write_factor est_c_w_f,estd_total_writes est_t_w,estd_total_write_factor est_t_w_f
3 from v$mttr_target_advice;
TAR_EST DIRTY_LIMIT EST_C_W EST_C_W_F EST_T_W EST_T_W_F
---------- ----------- ---------- ---------- ---------- ----------
60 5028 3762 .7376 3762 .7376
34 1000 5100 1 5100 1
68 6248 3762 .7376 3762 .7376
52 3808 3762 .7376 3762 .7376
45 2735 3845 .7539 3845 .7539
--mttr_target_for_estimate有一個值爲的最接近設定的目標時間30，以及由系統計算出的的target_mttr時間33
--同時也給出了幾組不同的mttr_target值及dirty_limit,cache_write,io 等來供DBA來選擇設定合適的mttr
五、兩個重要的視圖
v$instacne_recovery
v$mttr_target_advice