On Fri, Jan 31, 2003 at 11:30:18AM -0500, Steve Byan wrote:
> There's a notion afoot in IDEMA to enlarge the underlying physical 
> block size of disks to 4096 bytes while keeping a 512-byte logical 
> block size for the interface. Unaligned accesses would involve either a 
> read-modify-write or some proprietary mechanism that provides 
> persistence without the latency cost of a read-modify-write.

There probably ought to be a way of making the larger physical
size visible to systems that are willing to support larger
block sizes.  That way misaligned transfers would be far less
likely.

One problem to consider is that disks are still partitioned
on cylinder boundaries.  This is largely historic but isn't
this doen't actually make much sense, since the geometry
almost certainly varies across the disk and has to be faked
to fit the ATA CHS limits and (on PCs) the BIOS interface.

However what it does mean is that a partition could easily
not start on a 8 (512 byte) sector boundary.
So misaligned transefers are likely even if the filesystem
itself is using 4k blocks.

On a PC the partitioning will typically have the first one
starting in sector 63, and the others at multiple of 16065
sectors from the start of the disk).

This doesn't bode well for getting any aligned transfer
at all.

	David

-- 
David Laight: david@l8s.co.uk