newfs and mount vs. half-baked disks

[Kirk McKusick]

> Date: Mon, 10 Nov 2003 18:01:59 +1100 (EST)
> From: Bruce Evans <bde at zeta.org.au>
> To: Kirk McKusick <mckusick at mckusick.com>
> cc: arch at freebsd.org
> Subject: Re: newfs and mount vs. half-baked disks 
> X-ASK-Info: Whitelist match
> 
> On Sun, 9 Nov 2003, Kirk McKusick wrote:
> 
> > > From: Bruce Evans <bde at zeta.org.au>
> 
> > > The block count is in units of sector size, so disks much larger than
> > > 2TB can be supported by disklabel using (fake if necessary) sector sizes
> > > larger than 512.  File systems need to use similarly large block (fragment
>                                                                       ^^^^^^^^
> > > for ffs) sizes, and some patches are needed for reading superblocks if
>     ^^^^^^^
> > > the sector size is larger than 8K.  Since ffs uses a block size of 16K
> > > by default, a sector size of 16K are not unreasonable and this is
> > > sufficent for disks smaller then 64TB.
> >
> > Actually, FFS requires its fragment size be no smaller than the sector size
> > (since it presumes that it cannot do read/write in smaller than sector
> > sizes). So, on a 16K filesystem, you get 2K fragments. So your hack only
> > gets you to 8TB which is not going to last long at current disk growth
> > rates.
> 
> This point was noted in the underlined phrase.  The blocks size for ffs is
> actually the fragment size in this context.  So fragments would be as
> large as necessary (16K if that is the sector size), and the block size
> (he one given by newfs's -b parameter) would be larger.  A fragment size
> of 16K may even be the right size for very large disks.  My benchmarks
> say that 16K/8K block/fragment size is not much slower than 16K/2K on
> a 60GB disk, but 16K/16K and 32K/any are significantly slower.
> 
> Bruce

Using 16K/16K will not change the speed of operation appreciably, but
it will *more than double* the space required to hold the typical
FreeBSD filesystem (e.g., what would fit on a 60Gb 16K/2K filesystem
will require 120Gb on a 16K/16K filesystem). This happens because
most files on a typical filesystem are small. That is a poor tradeoff
in my opinion.

	Kirk McKusick