Subject: Re: non-512-byte-sector devices vs. UBC
To: None <>
From: Ignatios Souvatzis <>
List: tech-kern
Date: 06/14/1999 11:10:04
On Sun, Jun 13, 1999 at 04:07:43PM -0700, Erik E. Fair wrote:
> At 14:40 -0700 6/13/99, Christoph Badura wrote:
> > (Chuck Silvers) writes:
> >
> >>on a related note, it does seem odd that the current buf interface has
> >>the offset of the i/o (b_blkno) in sectors, but the size of the i/o
> >>(b_bcount) in bytes.  it would be more consistent for both offset and size
> >>to have the same units.  or are there devices that can read partial
> >>sectors, but only at the beginning of a sector?
> >
> >Tape drives do that.
> But no one uses tape drives through the block device; everyone uses the
> character ("raw") device, which does not go through the buffer cache.

Two things I want to mention:

1.) while raw tape drives do not use the buffer cache, raw ("character")
    accesses to block devices _do_ use the buffer header method to 
    describe to the device driver what to read/write.

2.) somebody might wish to use the block device to get read-ahead and better
    streaming of tape reads/writes...