On Wed, 26 Jul 2000, Daniel Phillips wrote:
> I don't know exactly what ReiserFS does - I just heard Hans mention the term
> 'tail merging' and I could see that it was a good idea.
>
I'll give the quick and dirty answer, if people want more details, let me
know. In 2.2, reiserfs_file_write deals directly with tails. It appends
to them if there is room in the packed block, or converts them if there
isn't. If reiserfs_file_write is called with a buffer size > 512 bytes,
it tries to write into full blocks instead of tails. This limits the
overhead when you cp/untar to create new files.
In both releases, there is locking on the tail to prevent races, and we
don't bother with tails on files > 16k (configurable).
For 2.4, the functions work like this:
reiserfs_get_block converts the tail into its own block and points
the buffer head at the new block.
reiserfs_readpage reads directly from the tail into the page, leaves the
buffer head mapped, and sets b_blocknr to 0.
reiserfs_writepage and reiserfs_prepare_write both check for mapped buffer
heads with a block number of 0 in the page. If found, they are unmapped.
Then block_write_full_page or block_prepare_write is called.
reiserfs_truncate deals directly with the tail. If the last block is
packed back into the tail, it is unmapped from the page cache.
reiserfs_file_release will check to see if the tail needs to be repacked,
and use truncate (without changing i_size) to pack the tail.
-chris
