ioctl for max_sectors

With what ioctl can I retrieve the max_sectors count of struct Scsi_Host_Template?

You don't, see /sys/block/sdX/queue/max_sectors_kb and max_hw_sectors_kb. The latter is the hardware advertised max value, the former is the one the block layer uses. You can only change the former, which can of course never exceed the latter.

Jens Axboe