Harddrive size being reported incorrectly?

[Mark Kirkwood]

Olivier Gautherot wrote:
> On Thursday 29 December 2005 18:34, Jerry McAllister wrote:
> 
>>>>[...]
>>>>When you slice and partition the drive, there will likely be a handful
>>>>of sectors that don't round out to an even value so those are dropped.
>>>>Then, when you do the newfs, some space is taken by the spare superblocks
>>>>and finally the system reserves 8%.     So, I would say you are getting
>>>>it all.
>>>
>>>289GB is before the 8% reservation. I actually turned that off with tunefs.
>>
>>I strongly suggest you do not do that - at least completely off.
>>Reduce it some, if you like, but keep some.
> 
> 
> I too strongly recommend you keep these 8% in.
> 
> The fact is that the space is not wasted: in the old days, it was meant
> to prevent the system from happily creating files until it dies - beyond
> 100%, files already opened could be written to but you could not create
> new files. Some kind of "soft landing". I suppose it is still the case.
> 
> Actually, a friend asked me a few weeks ago how the file system could
> reach 110% and he was speculating on how the system could use the
> swap partition to get to this level: it was not the swap partition but this
> extra space artificially held up.
> 
> You can safely and without afterthoughts let this 8% in.
> 

In addition, there are some potentially undesirable side effects that 
result from reduction - from the tunefs manpage discussing minfree:

<quote>
Note that lowering the threshold can adversely affect performance:

       o   Settings of 5% and less force space optimization to always be
           used which will greatly increase the overhead for file
           writes.

       o   The file system's ability to avoid fragmentation will be
           reduced when the total free space, including the reserve,
           drops below 15%.  As free space approaches zero, throughput
           can degrade by up to a factor of three over the performance
           obtained at a 10% threshold.

</quote>

The first effect can be mitigated by specifying '-o time', but I always 
leave it at the 8% default (fewer customizations for me to forget...).

Cheers

Mark