to just get the performance of aligned accesses, we dont need to modify
block sizes and such stuff. an an example, read the paper linked to from 
this; http://www.pdl.cmu.edu/PDL-FTP/stray/traxtent_abs.html
(brought to you by the same folks who did soft updates and raidframe)
happy hacking,
isildur

On Fri, 31 Jan 2003, Steve Byan wrote:

> 
> On Friday, January 31, 2003, at 12:59  PM, David Laight wrote:
> 
> > 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.
> 
> Yes, of course. But I asked with respect to an issue other than 
> performance.
> >
> > 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.
> 
> We understand that problem. It's just a performance issue. My concern 
> is that even if we handwave the performance issues, there's an 
> underlying semantic that would not be satisfied if we were to run 
> existing software, unmodified, on a disk with an underlying 4K sector 
> size.
> 
> Regards,
> -Steve
> --------
> Steve Byan <stephen_byan@maxtor.com>
> Design Engineer
> Maxtor Corp.
> MS 1-3/E23
> 333 South Street
> Shrewsbury, MA 01545
> (508) 770-3414
> 
>