Take it offline please (was [MacDV] Re: more about the inutility of defragmenting an OS X FS).

[E. Bond Francisco]

Peter, James,

Very enlightening discussion you two are having, but I think it's about 
time you took it offline (or out in the alley!)

Thanks,

Bond
-=-=-=-
On Dec 31, 2003, at 1:23 PM, James Asherman wrote:

>
> 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
>
>
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