[Ti] Journeling.

[Kynan Shook]

Yes, it does.

Some clarification on all the wonderful things added to 10.3's 
implementation of HFS+.

When the computer is about to perform an operation on the disk, it 
first writes what it's going to do to the journal, then marks the 
journal entry complete, then performs the task, then deletes the 
journal entry.  When 10.3 or later (or anything else that supports the 
journal) mounts a drive, it looks in the journal; if there are any 
entries, and they are marked complete, it performs the task in the 
journal.  If there are entries but they are not marked complete, they 
are ignored.  It's slightly slower to write something to disk, but 
startup is generally a fair amount faster (especially after a crash) 
since the startup disk doesn't have to be checked for consistency.

Something else that is kept track of is the "temperature" of each file. 
  The files that are used most often get moved to a special place on the 
disk, and are automatically defragmented as part of this process.  This 
space on the disk is allocated such that it is generally the part of 
the disk that is read at the greatest speed.  There are a number of 
limitations on what can be considered a hot file; it must be small, 
frequently read, and never written to, for example.  The hottest files 
are the ones that are read the most times - technically, it's the 
number of bytes read divided by the size of the file.

Additionally, there is also some defragmentation going on in the 
background.  If a file is read with more than 8 extents (fragments), 
then it is eligible to be defragmented, but only if it is also under 20 
MB, and there is enough free space available to defragment it.  If this 
turns out to be true, the file is copied to another location on the 
disk, and then is deleted from its original location.

Anyway, the bottom line is that Journaled HFS+ is something that most 
people should be using; the only people that should really consider 
turning it off are people who are not worried about data integrity, and 
require peak performance.  Generally, this means that only drives that 
are ONLY used for digital video should have it turned off; if you don't 
have a drive just for your digital video, then you probably don't use 
DV enough to justify turning it off anyway.  I can't really think of 
any other situation where it would be better to be left off.  Writing 
to the disk slows down by about 10% or so; so little of your computer's 
time is spent writing that turning it off won't make much of a 
difference, except for making reboots after a crash take much, much 
longer.

You can tell if you have Journalling enabled by either doing a Get Info 
on your hard drive, or by opening Disk Utility.  DU will also allow you 
to enable or disable Journalling (which includes the other features 
mentioned above) as you see fit.  No reformat is necessary, and a 
Journaled HFS+ drive can be used under an OS that doesn't know about 
Journalling; the older OS just doesn't take advantage of Journalling's 
features.  I believe that 10.3 automatically turned on Journalling, 
after 10.2.2 or so initially added support for it.

In case anybody is feeling sadistic, you can read enough about all of 
these things and more to make your very own disk utility at:
http://developer.apple.com/technotes/tn/tn1150.html

Be sure to read the summary at the bottom.  ;-)


On Aug 5, 2004, at 10:53 AM, silvo conticello wrote:

> I think that  journaling in Panther includes also some file 
> defragmentation on the fly. Or am I mistaken?