Blocking of Fixed-block Sequential Media.
Bitstream Devices.

P.Grosbol and D.Wells


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).