about fragmentation (of free disk space)

[Chris Jones]

Hi

> On 10 Oct 2015, at 7:45 p.m., Daniel J. Luke <dluke at geeklair.net> wrote:
> 
>> On Oct 10, 2015, at 7:28 AM, René J.V. Bertin <rjvbertin at gmail.com> wrote:
>> HFS+ is supposed to contain algorithms that limit file fragmentation, but without a background process that moves files (or file blocks), it cannot prevent free space fragmentation, just limit it. On a spinning disks that can become a limit on performance (I presume that theoretically the same applies to SSDs too)
> 
> 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.


Exactly. Also, access time does not change depending in what was accessed last, because nothing is physically moving as it is in a traditional HD. This is why fragmentation hurts HDs but is really not an issue  in SSDs. In fact, moving data around to 'fix' fragmentation (aka defraging) probably does more harm than good. The best you can do with an SSD is make sure trim is enabled, and then just let the drive sort itself out...

> 
>> and any process that requires contiguous files will ultimately fail if those cannot be obtained, regardless the underlying medium if it doesn't take that aspect into account.
> 
> Which API lets you know if you have a contiguous file or not / how do you ‘require’ a contiguous file on disk?
> 
>> Any thoughts on this, regardless of whether free disk space fragmentation is a real-world issue or not?
> 
> maybe something like hfsdbug could help you determine if it’s something to worry about or not?
> 
> http://osxbook.com/software/hfsdebug/
> 
> -- 
> Daniel J. Luke                                                                   
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