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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