This thread is a discussion related to a thread entitled "
Why Defrag" in the Mac OS X Tips & Hints forum by MicroMat Tech3. Moved to Tech Issues.
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What precisely is the status of this post?
Is it the advice/opinion of MicroMat? If so, why doesn't it appear in MicroMat's forum or on its web site?
Is it the personal advice/opinion of someone who also happens to be a MicroMat employee? If so, it should be clearly indicated.
In my opinion, there are serious problems with it, which should be pointed out, because it has been repeatedly referred to on a couple of discussion lists.
(1) Clear factual errors. For instance,
"The first three pieces (extents) of a Macintosh file are
recorded in the Catalog B-Tree. Any additional pieces are
recorded in the Extents B-Tree.[...] "
Actually, according to Apple tech note TN 1150 (an overview of HFS+), the catalog B-tree may contain up to 16 extents per file.
(2) Dubious assumptions (unwarranted, or clearly in contradiction with TN1150). For instance,
"I do not know the relationship between allocation block size and
clump size [...] "
"[...] if the relationship is linear.) [...] "
"[...] the new piece of Catalog B-Tree or Extents B-Tree [...]
is required to be written to disk space that is not only free,
but contiguous [...] "
The latter statement is directly contradicted by TN 1150:
"The special files [...] can appear in any order and are not
necessarily contiguous. [...]"
But the crux of the matter is here:
"[...] If the amount of contiguous free disk space is less than the
clump size, and a new piece of Catalog B-Tree or (more
likely) Extents B-Tree must be added to the disk, an older
piece of Catalog B-Tree or Extents B-Tree is overwritten. [...]"
This statement is simply outrageous. It assumes a fundamental error in the design of HFS+ or of Mac OS X which would put to shame a 3rd year comp. sci. undergrad, and which certainly did not exist in HFS. Such an assumption can only be accepted on the basis of strong evidence, and no such evidence is provided.
Yet, even granting all these flawed assumptions, the conclusion (that 85% of the disk should be free), still doesn't follow! According to the poster's own logic, on a 100GB volume, this irretrievable damage to the catalogue should occur when there is less than 200MB contiguous space free. That is, 0.2%, not 15%!