v15stanford.edu-File_System_Crash_Recovery

v15stanford.edu-File_System_Crash_Recovery - File System...

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http://w w w .stanford.edu/~ouster/cgi-bin/cs140-w inter12/lecture.php?topic=recovery January 11, 2012 File System Crash Recovery File System Crash Recovery Lecture Notes for CS 140 Winter 2012 John Ousterhout Readings for this topic from Operating System Concepts : Section 11.7 The problem: crashes can happen anywhere, even in the middle of operations: Lost data: information cached in main memory may not have been written to disk yet. E.g. original Unix: up to 30 seconds worth of changes Inconsistency: If a modification modifies multiple blocks, a crash could occur when some of the blocks have been written to disk but not the others. Examples: Adding block to file: free list was updated to indicate block in use, but file descriptor wasn't yet written to point to block. Creating link to a file: new directory entry refers to file descriptor, but reference count wasn't updated in file descriptor. Approach #1: repair during reboot Example: Unix fsck ("file system check") During every system boot fsck is executed. Checks to see if disk was shut down cleanly; if so, no more work to do. If disk didn't shut down cleanly (e.g., system crash, power failure, etc.), then scan disk contents, identify inconsistencies, repair them. Example: block in file and also in free list Example: reference count for a file descriptor doesn't match the number of links in directories Example: block in two different files
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v15stanford.edu-File_System_Crash_Recovery - File System...

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