[G4] defraging a Mac (the 85% rule)

[Wilkin, Wayne (Mass)]

What I was mainly getting to here is that the "Disks Directory" is the only
file structure that can't be fragmented. Brian, I thought your fridge
analogy was fine.

> ----------
> From: 	Brian Silverio
> Reply To: 	Power Macintosh G4 List
> Sent: 	Monday, April 5, 2004 10:15 AM
> To: 	Power Macintosh G4 List
> Subject: 	Re: [G4] defraging a Mac (the 85% rule)
> 
> Quoting "Wilkin, Wayne (Mass)" <Wayne.Wilkin at Staples.com>:
> 
> > I am probably going to hammered for this one but here goes. Besides
> > fragmentation there has also been a general consensus to always keep at
> > least 15% of your hard drive un-used. What does this have to do with
> > fragmentation. Ok so I can't remember were I read it but I did. Think it
> > was
> > Macfixit. But when a system writes to the hard drives directory the
> > directory needs to use continous disk space. The directory cannot be
> > fragmented. If a hard drive is so full that when rewriting directory
> > information it (the directory) can become corrupt. I look forward to
> others
> > views on this, Wayne.
> > 
> 
> For an analogy consider your refrigerator when you have the entire family
> coming
> over for dinner.  If the fridge is empty it is easy to find places to put
> the
> food.  You can even arrange it in some sequence.  But when the fridge is
> full it
> becomes difficult.  You have to move things around.  There may not be
> space for
> everything and so on.  (Watch how I get hammered for the analogy ;)
> 
> When you need space for a file the first choice is for contigious space.
> If a
> block of space is available large enough for the file that space is used.
> If
> you have a 200MB file and a 201MB space is used there is 1MB left over.
> As your
> drive fills up, it is POSSIBLE, that you will end up with many small
> chunks of
> space.  So when you go to save another 200MB file the only space available
> is
> 200 chunks of 1MB each.  This is what is meant by fragmentation.
> Please note that it is very unlikely that a 200MB file will be split into
> 200
> pieces.  The point is that it is now in several pieces and each time you
> load
> the file the time to load is increased.  
> 
> A disk access is split into five parts.  
> 1. The "path length" or amount of code traversed in the OS to perform the
> read.
> 2. The queue wait time
> 3. The time it takes to move the heads (seek) to the correct cylinder
> 4. The time it takes for the first sector to spin under the head
> 5. the time it takes for the data to be read
> 
> So for every physical segment of file, steps 1, 2, 3, and 4 are repeated.
> Step
> 5 remains essentually the same.  If the file was split into 200 pieces you
> would
> notice the difference.
> 
> This explanation is simplified for clarity.
> 
> 
> Brian Silverio
> 
> 
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