Kernel Log – Coming in 2.6.31 - Part 3: Storage and file systems

"Major changes, in the form of a 350 KB patch, should significantly improve the experimental file system's performance by using 'mixed back references' in many areas. The patch does, however, involve a change in the structure of the file system on the storage media ('on disk format'). Kernel versions containing the new Btrfs code deal with the requisite conversion from old to new format automatically the first time the file system is mounted. However, Linux versions with older Btrfs code will thereafter no longer be able to mount file systems which have been modified by the new code.

"This is flagged up clearly in the commit comments and Git pull request. Kernel developers usually try to avoid such situations, even with experimental file systems – the result of this change is that hardy users who choose to use Btrfs as their root file system will find themselves unable to start older kernel versions to deal with errors should the need arise. Indeed this is precisely the misfortune that befell Linus Torvalds, who was distinctly unimpressed."
This has been an amazing series, a must-read for kernel geeks everywhere!-- ed.

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