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8.5.1.19: MTSETDRVBUFFER

zftape abuses this command to set the number of dma buffers it consumes for its cyclic read/write buffer list.

ftape needs at least 3 dma buffers to work reasonably well. The maximum allowed number of dma buffers is 16 (randomly chosen value). Keep in mind that kernel memory cannot be swapped out. This feature is limited by the physical memory fragmentation as zftape needs a continuous block of at least 32k aligned at 32k boundary. The return value is -ENOMEM if the driver wasn't able to allocate the desired size. In this case it allocates as much as it can.

Example
set the number of dma buffers to 10. As each dma buffer has a size of 32kb this results in 320kb of memory being allocated for ftape.

mt -f /dev/nqft0 setdrvbuffer 10

References
see Swapout


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