Re: blocksizes

[Michael van Elst <mlelstv%serpens.de@localhost>]

On Mon, Feb 01, 2010 at 07:48:01PM -0500, Thor Lancelot Simon wrote:
> On Mon, Feb 01, 2010 at 10:59:37PM +0000, Michael van Elst wrote:
> > 
> > It still wouldn't be perfect. Block size is one thing, block alignment
> > the next. A filesystem with 1k blocks (filesystem units == clusters)
> > has problems to work on disks with 2k blocks (device units == sectors).
> > The only solution is expensive buffering and read-modify-write updates.
> 
> Aren't we in fact going to run into this with 512-byte FFS fragments
> and disks with 4K sectors?

Yes. FFS with 512-byte fragments and 4K sectors simply does not work.
There is no code that handles this and it would be slow anyway.

Fragments must be at least as large a disk sector.

That already happens almost automatically because of newfs defaults:

  file system size  frag-size
  < 20 MB           0.5 KB
  < 1024 MB         1 KB
  >= 1024 MB        2 KB

Maybe there should be another level (1TB?) that choses 4K fragments
(and 32KB blocks) by default but newfs will already refuse to create
a filesystem with fragments (or blocks) smaller than sectors.

And then there is the 64K MAXPHYS limit on the other end.


Greetings,
-- 
                                Michael van Elst
Internet: mlelstv%serpens.de@localhost
                                "A potential Snark may lurk in every tree."