logs vs journaling
一直没想过log文件系统和journaling文件系统之间有什么区别。实际上根本就不懂，以为是一回事。其实区别很大，别看中文往往都翻译成日志文件系统
There have been several approaches of implementing
file system consistency; log structured
file system [11] or journaling file system [2].
In log structured file system, all operations are
logged to disk drive. File system is structured
as logs of consequent file system update operations.
Journaling is to store operations on
separate journal space before updating the file
system. Journaling file system writes twice (to
journal space and file system) while log structured
file system does write once. However
journaling approach is popular because it can
upgrade existing non-journaling file system to
journaling file system without losing or rewriting
the existing contents.