Btrfs Power-Fail Tolerance

Posted Jul 3, 2013 8:27 UTC (Wed) by nirbheek (subscriber, #54111)
Parent article: A flash filesystem tuning guide

The test for power-fail tolerance on Btrfs (Section 5.2.2) is suspect. Their criteria for corruption seems to be whether `fsck` finds errors, not whether files were corrupted on-disk.

Btrfs's fsck is distinct from traditional fsck tools, and is not meant to be used at boot time to check for consistency. AFAIK, it will warn on inconsistencies that would be automatically corrected when the filesystem would get mounted, and seeing `btrfsck` errors after a dirty reboot is expected behaviour.

If my understanding is correct, then that section needs to be revised.

Btrfs Power-Fail Tolerance

Posted Jul 3, 2013 11:04 UTC (Wed) by tialaramex (subscriber, #21167) [Link] (1 responses)

The authors write that

"fsck finds errors which it is not able to auto-recover"

Is it the case that when Btrfs's fsck warns "on inconsistencies that would be automatically corrected [if you hadn't run fsck]" it requires manual confirmation from the user? If so what's the justification for this bizarre design choice?

If not, then the authors have done exactly as they said, detecting situations where auto-recovery isn't possible. For all but the most technical users a single prompt asking some low-level jargon based question like e.g.

"Bazingle nerrubishment fondangoloro 564600? Y/n?"

is enough for them to conclude that all their data is forfeit and they'll need to start over (hopefully from a backup, if they kept backups) unless somebody technical is available to help them push the enter key and see what happens next. A filesystem or filesystem configuration that's _more likely_ to cause these failures will result in more warranty calls, less customer satisfaction and higher costs, so it's bad, even if it's technically very clever indeed.

Btrfs Power-Fail Tolerance

Posted Jul 3, 2013 16:51 UTC (Wed) by Thom (guest, #73471) [Link]

From the btrfs wiki, "The btrfsck tool in the git master branch for btrfs-progs is now capable of repairing some types of filesystem breakage. It is not well-tested in real-life situations yet."

This paper touches on the important points for an embedded file system - it seems to me that btrfs is more of a desktop or server alternative. For embedded use, protecting file system metadata and allowing drives to mount is quite important in "real-life situations".