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Linux Magazine: Journaling Filesystems; The Future Of Storage Under Linux

Oct 29, 2000, 16:48 (6 Talkback[s])
(Other stories by Moshe Bar)

"Journaling filesystems are superior to static filesystems when it comes to guaranteeing data integrity and even when it comes to flat-out filesystem performance. Replacing the ext2fs static filesystem with a journaling filesystem will ultimately be a big win for all Linux users."

"When a Linux box is rebooted, it runs a program called fsck (filesystem consistency check) that walks through the entire filesystem, validating all entries and making sure that blocks are allocated and referenced correctly. fsck would find the corrupted directory entry and attempt to repair it. However, there is no guarantee that fsck will actually be able to repair the damage. ... On a machine with many gigabytes of files, fsck can run for up to 20 minutes per filesystem."

"Journaling and logging filesystems solve many of these problems. Journaling and logging filesystems can either keep track of the changes to a file's "metadata" (information such as ownership, creation dates, and so on), or to the data blocks associated with a file, or to both, rather than maintaining a single static snapshot of the state of a file. ... Journaling and logging filesystems get around the problem of inconsistencies introduced during a system crash by using the log. Before any on-disk structures are changed, an "intent-to-commit" record is written to the log. The directory structure is then updated and the log entry is marked as committed. ... After a crash, filesystems can come online almost immediately because only the log entries after the last checkpoint need to be examined. Any changes in the log can be quickly "replayed," and the corrupted part of the disk will always correspond to the last change added to the log. The log can then be truncated since it will be invalid, and no data is lost except for any changes that were being logged when the system went down."

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