Fast commits for ext4

This is actually not quite correct. Delayed allocation just means that write(2) stores data in the page cache without actually allocating blocks on disk. This also means that the journalling machinery is completely ignorant of the write at this moment. Later, when VM decides to write out dirty pages from the page cache, filesystem allocates blocks for the pages and it is only at this point that there are filesystem metadata changes that are journalled. So it isn't true that "delayed allocation may delay writing of the journal".