DISC(4)             UNIX Programmer's Manual              DISC(4)



NAME
     disc - direct disc access

HP-UX COMPATIBILITY
     Level:    HP-UX/RUN ONLY

     Origin:   HP

DESCRIPTION
     This page describes the actions of the  general  HP-UX  disc
     drivers  when  referring  to  a  disc as either a blocked or
     unblocked (raw or character special) device.

     Block special files access discs  via  the  system's  normal
     buffering  mechanism.   Buffering is done in such a way that
     concurrent access through multiple opens or  mounts  of  the
     same physical device do not get out of phase.  Block special
     files may be read and written  without  regard  to  physical
     disc records. Each I/O operation results in one or more log-
     ical block (1K bytes) transactions.

     There is also a _r_a_w interface via a character  special  file
     which  provides for direct transmission between the disc and
     the user's read or write buffer.  A  single  read  or  write
     operation results in exactly one transaction.  Therefore raw
     I/O is considerably  more  efficient  when  many  bytes  are
     transmitted  in  a  single  operation  because  blocked disc
     access requires potentially several  transactions  and  does
     not transmit directly to user space.

     In raw I/O, there may be implementation  dependent  restric-
     tions  on the alignment of the user buffer in memory.  Also,
     each transfer must occur on a sector boundary and must  read
     a  whole  number  of sectors.  The sector size is a hardware
     dependent value.

     Each raw access is independent of other raw accesses and  of
     block accesses to the same physical device.  Thus, transfers
     are not guaranteed to occur in any particular order.

     In both raw and blocked I/O, each operation is completed  to
     the  device  before  the call returns.  In addition, blocked
     I/O potentially does a one (or more) block read-ahead.

     The name of a raw device (its character special  file  name)
     is  typically  the  same  as  the  name of the corresponding
     blocked device (its block special file name)  prefixed  with
     an "r".

SEE ALSO
     intro(4), mkdev(8), mknod(8), and the _H_P-_U_X _S_y_s_t_e_m  _A_d_m_i_n_i_s_-
     _t_r_a_t_o_r _M_a_n_u_a_l included with your system.



Printed 12/17/84                                                1






DISC(4)             UNIX Programmer's Manual              DISC(4)



WARNING
     On some systems, having both a mounted  file  system  and  a
     block  special  file  open  on the same device is asking for
     trouble; this  should  be  avoided  if  possible.   This  is
     because  it  may  be possible for some files to have private
     buffers in some systems.

     Like discs, the cartridge tape units in command set 80  disc
     drives  are  also  accessed as blocked or raw devices.  How-
     ever, using a cartridge tape as a file system  will  quickly
     destroy the tape drive.












































Printed 12/17/84                                                2