[MacDV] Re: more about the inutility of defragmenting an OS X FS.

[James Asherman]

On Wednesday, December 31, 2003, at 03:56  PM, Peter van der Linden 
wrote:

> On Dec 31, 2003, at 11:02 AM, Mark M. Florida wrote:
>
>> It's certainly true that modern drive and OS technology lessens the 
>> need for defragging, but that's for what would be considered "normal" 
>> usage like e-mail, word processing, and even editing small images.
>
> Neither the kernel, nor the disk i/o subsystem has any idea what 
> applications are being run. They just see a stream of I/O requests.  
> Most applications are i/o bound (video encoding is an exception, but 
> video encoding is not a real time constrained operation), so most 
> applications look the same to the OS and i/o subsystem. I.e. disk i/o 
> requests come in asynchronously, and are serviced asynchronously.

Gibberish in the context of this discussion.

>
> You don't and can't know how the blocks are laid out on physical disk. 
>  You can't know it  because that information doesn't leave the disk 
> controller.  The controller maintains a fiction of putting the blocks 
> where you ask, but it actually puts the blocks wherever it wants to.  
> (And the algorithms that the controller uses in its attempts to 
> optimize logical block placement are highly guarded secrets, too). The 
> nice GUI maps can only show you the fiction, not the physical 
> on-platter reality.  For that reason alone, rearranging disk blocks to 
> make the map look pretty, does nothing for performance.

I',
m
not stupid. I know how a disc works and what are the illusions and what 
are the realities.
  The reality is we need 10's of gigs all ina row so that we miss not 
1/60th of a second of video looking for someplace new to put it. And 
when playing it back after messing with it (more files distributed by 
the software and discs)  the demand for fast location and processing is 
even more important. My stuff has to deliver 3.6 megabytes of carefully 
sequenced material in a continuous stream for two hours. IT gotta be 
clean man!

>
> On top of that, you have all the buffering and cacheing done by the 
> kernel I/O subsystem.  And on top of that, you have unrelated disk 
> accesses done by the OS, such as swap, tempfs, journalling, and i/o 
> for other processes.

That is why we have dedicated video discs. It bugs me that even the 
muzak comes from the
boot disc but it makes it quicker to reformat the video scratch/capture 
disc. Muzak is like 4 gigs.

> These are going to put the disk heads wherever they want, and attempts 
> to make files contiguously allocated therefore yield insignificant 
> performance improvements except possibly under pathological > conditions.

Realities of capturing  and playing back video music and rendered 
filles have not changed to the point of pathology. Or people with iMacs 
wouldn't run into brick walls with this stuff.


>
> A separate drive for video is a good idea, but because of dedicated 
> performance, not because of fragmentation.
That one never gets fragmented much. It has no chance.
It's the boot drive where I play. I dl a game, a song, an image,I 
render an animation, I write a letter , I use iPhoto and then delete 
images, all this stuff decreaes contiguous space ( desired for large 
media filles and related renders) and occaisionally the startup comes 
into play in work. Hence it is kept optimized.

>   If you're a true believer in defragmentation then putting your video 
> on a separate partition on the same drive is a terrible idea - it 
> guarantees that all other references to disk will reposition the > heads.

No partitions, because of what you imply. It splits the heads too, and 
then access and performance suffer. I tried it. No good.


>
> There's a good layman's intro to modern disk technology at
> http://www.acmqueue.org/modules.php?name=Content&pa=showpage&pid=46
>
> But, hey, prove me wrong, show me some data that supports claims that 
> disk defragmentation improves application performance in MacOS 10.3  
> Tell me how I can reproduce this alleged performance drop on one of my 
> own systems.

Use Xbench. record your score.
Use Speed disk and Disc warrior.
Reboot.
  Use Xbench again. Compare scores.   Arbitrary but consistent.

>
>      Peter
>
>

PS edited 47 minutes  since last message. Seagate barracudas. A oK
Happy New Fear
Jim