tunefs question

[Brooks Davis]

On Thu, Jun 07, 2007 at 05:04:09PM +0200, Richard Noorlandt wrote:
>  Hi everybody,
> 
>  While reading a bit about tunefs I noticed that UFS reserves 8% of the drive
>  space for the root user and the system. However, I don't really understand
>  what this space is actually used for. From the tunefs man page I understand
>  that it is primarily used to guard against fragmentation, and that's about
>  it. Is this the only thing that the reserved space is used for?
> 
>  I'm building a large array for my fileserver, and it actually hurts a bit to
>  see that so much space is "wasted" without a very clear reason to do so.
>  Especially because the data on the array won't be modified very often, it
>  appears to be quite a lot of disk space just to prevent fragmentation. Does
>  anybody have some more information on this?
> 
>  And while I'm at it: what is the effect of the expected average file size
>  option? What are the benefits and dangers of tweaking it? From the FreeBSD
>  handbook I understand that the FS actually optimizes itself as time passes,
>  but that's about all that's said about it.

The answer I've heard in the past is that the performance of the block
allocation algorithms requires that most of the time an attempt to
allocation from a cylinder group will succeed.  If you reduce the free
space too much that will not hold true anymore and thus your performance
will suffer.  8% is actually a reduction from the historical 10%.  In
practice, the only way to decide what's right for your system is
testing, but that's hard on a large system since it will take days to
fill the disk.

-- Brooks
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 187 bytes
Desc: not available
Url : http://lists.freebsd.org/pipermail/freebsd-fs/attachments/20070607/d069e9cb/attachment.pgp