about fragmentation (of free disk space)

Justin C. Walker justin at mac.com
Sat Oct 10 12:48:06 PDT 2015

On Oct 10, 2015, at 12:06 , René J.V. Bertin wrote:

> On Saturday October 10 2015 14:45:43 Daniel J. Luke wrote:
>> random access time for a SSD is 1-3 orders of magnitude less than for a rotational drive.
>> As with anything, you need to measure ‘real world use’ to be certain, but it’s probably not an issue for SSDs at all.
> I'm not expecting it to be in real world usage, of course. OTOH, frequent defragmenting is probably not a good idea on SSDs.
>> Which API lets you know if you have a contiguous file or not / how do you ‘require’ a contiguous file on disk?
> Not sure, but I seem to recall it is possible. That said, I may just have observed that in ... defragmentation utilities (which tend not to work on files they cannot move to a contiguous bit of disk space).

I'm not convinced that defragging is a necessity on modern (rotational) drives, and for SSDs I don't think it makes sense As Daniel suggests).  These days, don't most drives do a lot of caching?  Also, OS X in recent incarnations does a pretty good job of detecting when you are streaming a file and doing a lot of prefetching.


PS: A better place to ask this is on the Darwin Kernel mailing list (hosted by Apple).  There are disk/FS experts that may be able to give you some pretty good answers.

Justin C. Walker, Curmudgeon at Large
Institute for the Absorption of Federal Funds
I'm beginning to like the cut of his jibberish.

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