Journaled Soft-Updates, Dr. Kirk McKusick, BSDCan 2010

Adding "journaling lite'' to soft updates and its incorporation into the FreeBSD fast filesystem

Because soft updates prevent most inconsistencies, the journaling need only deal with tracking those inconsistencies that soft updates fails to address. Specifically, the journal contains the information needed to recover the block and inode resources that have been freed but whose freed status failed to make it to disk before a system failure. After a crash, a variant of the venerable fsck program runs through the journal to identify and free the lost resources. Only if a corruption of the log is detected is it necessary to run background fsck. The journal is tiny, 16Mb is usually enough independent of filesystem size. Although journal processing needs to be done before restarting, the processing time is typically just a few seconds and in the worst case a minute. It is not necessary to build a new filesystem to use soft-updates journalling. The addition or deletion of soft-updates journaling to existing FreeBSD fast filesystems is done using the tunefs program.

http://www.bsdcan.org/2010/schedule/e...

• Category

  Science & Technology

[bsdconferences]

When we grabbed the existing flag, we created a new one with the same semantics. To verify if any older kernel has been run, it will now be necessary to check whether either of the original or this new flag have been cleared, but that is still a single test as both flags are in the same word.

[ResunaTrue]

Did you consider writing the unlinked inode information directly to the journal instead of a linked list off the superblock?

[bsdconferences]

Unlinked files may remain in that state for days. The journal only keeps short-term changes to the file system. Since it is used in a circular order, any long-term events stored in it would prevent it from being reused. Additionally the journal could be exhausted by creating an unbounded number of unlinked files. The linked list is an extensible data structure that cannot be exhausted.

[Diego Galizzi]

Dr. McKusick is god... period...

[ResunaTrue]

Thanks, very very cool.

[ResunaTrue]

Nice win on the btree flag, did you make some provision for additional flags in the future?

[ResunaTrue]

Looked like he actually dropped the mic there.

[Pat Gunn]

Lost audio around 17 minute mark, comes back around 17:20