Blocking of Fixed-block Sequential Media.
and
Bitstream Devices.
P.Grosbol and D.Wells
1994-May-10
Introduction
A number of high density storage media (e.g. optical disks and helical
scan tapes) is of significant interest of exchange of large volumes of
astronomical data in FITS format. Many controllers and devices for these
media can only access data in blocks of fixed length, typically 2**n bytes.
Thus, a special blocking agreement for FITS files is required for this
type of media.
Blocking for fixed-block media.
For fixed-block sequential media, FITS files consisting of an integer
number of 2880 byte logical records should be regarded as a bitstream
written out with the fixed blocking size of the medium with the last
block being padded out with zeros to the length of the fixed blocks.
After detection of an end-of-file on reading, any incomplete logical
FITS record should be disregarded. This proposal applies to optical
disks (accessed as a sequential set of records), QIC format 1/4inch
cartridge tapes and Local Area Networks.
The fixed-block blocking proposal conforms to the general rules
for blocking of FITS files (Grosbol et al. 1988, Astron. Astrophys.
Suppl. 73, p359) using a blocking factor of 2**n/2880 for media
with a fixed block size of 2**n bytes.
Blocking for bitstream devices.
For bitstream devices, FITS files should be written with a blocking
factor of one i.e. with fixed blocks of 2880 bytes corresponding to
the logical record size. This proposal applies to FITS files written
to logical file systems.
Blocking for variable-block media.
For variable block length sequential media, FITS files may be written
with an integer blocking factor between 1 and 10 including as specified
in the blocking agreement for 1/2 inch 9 track tapes. This proposal applies
to FITS files written to DDS/DAT 4mm cartridge tapes and
8mm cartridge tape (Exabyte).