The ext3 file system layout is the same as the ext2 file system layout with a journal added.
The basic structure looks like this:
[Boot Block][Block Group 0][...][Block Group N]
A block group looks as follows
[Super Block][Group Descriptors][Data Block Bitmap][Inode Bitmap][Inode Table][Data blocks]
The super block and both bitmaps are 1 block each, the rest are n blocks.
The journal just insures system call level protection of writes. When you use the write system call either the write will finish completely, or the data will remain as it did before the write. Partial writes are discarded.
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