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8.5.1.15: MTSETBLK

zftape only. Set block length to count bytes.

The default block size of zftape is 10kb which matches the default of the GNU tar program. The argument count has to be a multiple of 1024 (including 0). Due to the QIC-113 standard (http://www.qic.org) a maximum block size of 62kb is allowed. An argument of `0' means to switch to variable block size mode. This is implemented in zftape by setting the internal block size variable zft_blk_sz to `1' and thus has some unexpected side affects on the MTFSR, MTBSR and MTSEEK ioctls which interpret their arguments in units of zft_blk_sz bytes, i.e. they take their argument as a byte count when zftape is in variable block size mode.

When writing a volume, the block size used for the volume is stored in the corresponding volume table entry, (see MTIOCVOLINFO), because the driver needs to remember the block size of a tape volume when using compression.

When reading a volume one has to use the block size the volume was created with. There is no need use the MTSETBLK ioctl again but the read count has to be a multiple of the original block size.

Example
set the block size to the maximum allowed

mt -f /dev/nqft0 setblk 63488

Example
switch to variable block size mode; not possible when using compression

mt -f /dev/nqft0 setblk 0

References
see Tape blocks
see MTFSR
see MTBSR
see MTSEEK
see Compression


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