Re: Disks w/non-512-byte sectors?

[John Nemeth <jnemeth%cue.bc.ca@localhost>]

On Jan 18,  9:08pm, Greg Troxel wrote:
} mlelstv%serpens.de@localhost (Michael van Elst) writes:
} > jdbaker%mylinuxisp.com@localhost ("John D. Baker") writes:
} >
} >>First, can anyone point me to message threads about how modern 4K-sector
} >>disks are to be handled?  I must admit some confusion when reading about
} >>4K native sectors, but having to deal with a 512-byte emulation mode.
} >>At what point does the emulation get out of the way and allow one to addr=
} ess
} >>the native 4K sectors?
} >
} > With 512-byte emulation mode you do not address 4K sectors, that's
} > the whole point. You can treat the disks just as disks with 512-byte
} > sectors and they will "work". However, you should still arrange the=20
} > data on the disks so that accesses are aligned to the 4K sectors,
} > otherwise you will see performance penalties.
} >
} > In particular:
} >
} > - start partitions on 4K (or multiple) boundaries. A common value
} >   is to use 1M alignment (which wastes a few sectors).
} > - use filesystem block sizes of 4K (or multiple). With FFS that's
} >   the _fragment size_. You need to tell newfs about this, because
} >   it can only see the emulated block size.
} 
} Agreed on all of that, but to respond to John's question: the disk has
} 4K sectors on the media, but has an interface that acts like 512B
} sectors.  So if you do a write which is aligned to start on 4K and is a
} multiple of 4K, it will be a straightforward write of some number of
} underlying whole sectors.  If the write is misaligned or fractional, it
} will result in read-modify-write by the disk controller.
} 
} In particular, the common PCism of starting at sector 63 (1 floppy track

     Uh, starting at sector 63, has nothing to do with floppies.  The 1.44MB
floppy had 18 sectors per track.  I'm not aware of any floppies that had
63 sectors per track.

}-- End of excerpt from Greg Troxel