File System Crash Recovery

Lecture Notes for CS 140
Spring 2020
John Ousterhout

  • Readings for this topic from Operating Systems: Principles and Practice: Chapter 14 up through Section 14.1.
  • The problem: crashes can happen anywhere, even in the middle of critical sections:
    • 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 affects 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 inode wasn't yet written to point to block.
        • Creating link to a file: new directory entry refers to inode, but reference count wasn't updated in inode.
      • The block cache may reorder writes.
    • Ideally, we'd like something like an atomic operation where multi-block operations happen either in their entirety or not at all.

Approach #1: check consistency during reboot, repair problems

  • Example: Unix fsck ("file system check")
    • During every system boot fsck is executed.
    • Checks to see if system was shut down cleanly; if so, no more work to do.
    • If system 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 an inode doesn't match the number of links in directories
    • Example: block in two different files
    • Example: inode has a reference count > 0 but is not referenced in any directory.
  • Limitations of fsck:
    • Restores disk to consistency, but doesn't prevent loss of information; system could end up unusable.
    • Security issues: a block could migrate from the password file to some other random file.
    • Can take a long time: can't restart system until fsck completes. As disks get larger, recovery time increases.

Approach #2: ordered writes

  • Prevent certain kinds of inconsistencies by making updates in a particular order.
    • For example, when adding a block to a file, first write back the free list so that it no longer contains the file's new block.
    • Then write the inode, referring to the new block.
    • What can we say about the system state after a crash?
    • In general:
      • Never write a pointer before initializing the block it points to (e.g., indirect block).
      • Never reuse a resource (inode, disk block, etc.) before nullifying all existing pointers to it.
      • Never clear last pointer to a live resource before setting new pointer (e.g. mv).
  • Result: no need to wait for fsck when rebooting
  • Problems:
    • Can leak resources (run fsck in background to reclaim leaked resources).
    • Requires lots of synchronous metadata writes, which slows down file operations.
  • Improvement:
    • Don't actually write the blocks synchronously, but record dependencies in the buffer cache.
    • For example, after adding a block to a file, add dependency between inode block and free list block.
      • When it's time to write the inode back to disk, make sure that the free list block has been written first.
    • Tricky to get right: potentially end up with circular dependencies between blocks.

Approach #3: write-ahead logging

  • Also called journaling file systems
  • Implemented in Linux ext3 and NTFS (Windows).
  • Similar in function to logs in database systems; allows inconsistencies to be corrected quickly during reboots
    • Before performing an operation, record information about the operation in a special append-only log file; flush this info to disk before modifying any other blocks.
    • Example: adding a block to a file
      • Log entry: "I'm about to add block 99421 to inode 862 at block index 93"
    • Then the actual block updates can be carried out later, in any order.
    • If a crash occurs, replay the log to make sure all updates are completed on disk.
    • Guarantees that once an operation is started, it will eventually complete.
    • Problem: log grows over time, so recovery could be slow.
    • Solution: occasional checkpoints:
      • Record current log head
      • Flush all dirty blocks to disk
      • Once this is done, the log can be cleared up to the recorded position
    • Typically the log is used only for metadata (free list, inodes, indirect blocks), not for actual file data.
  • Logging advantages:
    • Recovery much faster.
    • Eliminate inconsistencies such as blocks confused between files.
    • Log written sequentially, so log writes are faster (no seeks).
    • Metadata writes can be delayed a long time, for better performance.
  • Logging disadvantages:
    • Synchronous disk write before every metadata operation.
  • Solution: delay log writes
    • Assign log positions immediately, but don't write
    • Mark each cache block with latest log position related to that cache block
    • Before evicting cache block, flush log
    • This separates durability from consistency

Remaining problems

  • Crashes can still lose recently-written data if it hasn't been flushed to disk.
    • Solution: apps can use fsync to force data to disk.
  • Disks fail
    • One of the greatest causes of problems in large datacenters
    • Solution: replication or backup copies (e.g., on tape)
  • Conclusions:
    • Interesting tradeofs between performance, durability, and consistency
    • To get highest performance, must give up some crash recovery capability.
    • Must decide what kinds of failures you want to recover from.