Oh my god! They‘ve done it! As it seems, Apple has introduced a really cool under-the-hood feature into Mac OS X 10.3 (you know, the black cat thing). It is some kind of auto-defragmentation within the HFS+ file system.

As documented in this Ars Technica Forum Thread, the source code for this can be reviewed in the recently posted Darwin sources.

Heres how it works: files under 20 MB in size get checked for fragmentation on open and if they are too distributed, they get reallocated. So a contiguous block of data is allocated, the data is copied over and the old data gets cleared afterwards. The result is a nicely ordered file. No need for disk doctors anymore as this seems to be a viable solution to the problem of scattered data.

Here is the fun part: the source code itself contains a nice illustration of how this thing works. It‘s so great: whirr… shhwip! :>POOF!<: *gleam*

/*
 * Relocate a file to a new location on disk
 *  cnode must be locked on entry
 *
 * Relocation occurs by cloning the file‘s data from its
 * current set of blocks to a new set of blocks. During
 * the relocation all of the blocks (old and new) are
 * owned by the file.
 *
 * -----------------
 * |///////////////|
 * -----------------
 * 0               N (file offset)
 *
 * -----------------     `´`´`´`´`´`´`´`´`
 * |///////////////|     }    whirr...   {     STEP 1 (aquire new blocks)
 * -----------------     `´`´`´`´`´`´`´`´`
 * 0               N     N+1             2N
 *
 * -----------------     -----------------
 * |       ////////| ===}|///////        |     STEP 2 (clone data)
 * -----------------     -----------------
 * 0               N shhhwip!            2N
 *
 *                       -----------------
 *     :>POOF!<:         |////*gleam*////|     STEP 3 (head truncate blocks)
 *                       -----------------
 *                       0               N
 *
 * During steps 2 and 3 page-outs to file offsets less
 * than or equal to N are suspended.
 *
 * During step 3 page-ins to the file get supended.
 */

I love it.