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記日前硬盤故障
先描述一下我的計算機概況：
500G硬盤，前大約100G是OEM的Windows7及其預置恢復分區（sda1-sda3），後邊全給了linux用。除了/boot，文件系統格式全是ext4，/home（sda6）有200G+。
發行版是arch，引導器是grub2。
某日，需要進Windows做點事，於是重啓電腦。到了引導項選擇那裏，由於我設置的是隱藏菜單（默認進arch），故而需要按ESC使菜單顯示出來。然而，我手快了些，在5s的倒計時開始前就按了ESC。然後只見硬盤持續亮着，引導菜單沒有出現。等了一會，實在不耐煩，於是長按電源，再開。這次刻意等到讀秒開始之後再按的ESC，引導項選擇也出現了，選擇Windows，確認。Windows的啓動用了很長時間，輸入密碼之後那個“歡迎”也持續旋轉很久。我以爲是上次安裝了一些更新所致，也沒有在意，之後使用沒什麼奇怪現象。
等到處理完，重啓進Arch時候，看到問題了：一直在報sda6一些塊讀取錯誤。看到這裏，大概判斷是硬盤有了壞塊，於是長按電源鍵，關掉了計算機。再次啓動時候是用U盤引導（上次給人裝Arch時候直接把鏡像dd到U盤），繼續看到sda6某某塊讀取出錯，不過這次倒是進了live系統。
然後用badblocks檢測，發現還真是有，主要分佈在sda6上，sda7也有少量。這下就令人着急了——數據可是無價的。幸好手上有個1T且只裝了一半的移動硬盤，可以將數據轉入。
然而，嘗試只讀掛載sda6的時候，提示superblock讀取失敗，拒絕掛載。上網一查，發現ext4會創建備份的superblock，於是按照網上說的，掛載時加上-o sb=xxx參數指定副superblock的塊號。這裏我man了一下mount，發現這個參數是以1K爲標準的。我創建的文件系統塊大小是4K，所以第32768塊的備份superblock應該用-o sb=131072參數。
這樣，連起來的掛載命令就是
mount -r -o sb=131072 -t ext4 /dev/sda6 /mnt/sda6
滿懷期待得讓命令執行，卻再次得到信息說“掛載失敗，可能是錯誤的文件系統類型”。看到這點時候真的有些慌，因爲我確認它是ext4，而掛載時候卻告訴我失敗，莫非數據真的無法挽救？之後又嘗試用其他的副superblock，結果卻沒有什麼變化。
這時候我抱着試試看的心態敲了一下dmesg，在最下邊看到journal讀取失敗，於是拒絕掛載。心情一下清平了，再掛載時候加上norecovery參數，掛載成功，備份數據。
最終的命令如下：
mount -r -o sb=131072,norecovery -t ext4 /dev/sda6 /mnt/sda6
慘痛的經歷再次說明經常備份數據的重要性。
以及，若有遇到類似情況的，望可告知產生原因是什麼……
（某想不通究竟是因爲斷電導致的還是確實是grub的bug……更不想再試試……）
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