CS 481: Operating System Principles
Spring ’09
Quiz #6: Filesystems
1. Journaling Filesystems (25 pts)
Consider a filesystem in which updates are made in the following order:
1. Free list update
2. File data block update
3. File inode update
(Your answers should be no more than 2 sentences.)
1.1) What is the result of a crash between events 1 and 2? (5 pts)
Free list reflects a block in use, but since inode hadn’t been updated, block is “leaked”,
i.e. disk storage is wasted. There are no functional inconsistencies.
1.2) What is the result of a crash between events 2 and 3? (5 pts)
A disk storage block is “leaked”, and the time taken to update the block is also wasted.
There are no functional inconsistencies.
1.3) In this filesystem, what would be the purpose of a consistency checker? (5 pts)
The consistency checker’s job is to make sure that blocks shown as “in use” by the free
list are actually in use by files as reflected in their inodes.
1.4) What is the single biggest disadvantage of the consistency checker? (5 pts)
The consistency checker is slow because it has to scan all files on the disk.
1.5) How do journaling filesystems address this deficiency? What is the purpose of the
journal or log? (5 pts)
Journaling addresses this slowness by restricting scans to only transactions that were
in progress when the system crashed. The log or journal maintains the list of “in-
progress” transactions.
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