Hi all, I am building an experimental device mapper target which is supposed to receive all TRIM requests (REQ_DISCARD). Irrespective of whether underlying actual physical block device supports discards or not, my device mapper target should receive REQ_DISCARD bios. In my target, I set: ti->num_discard_requests = 1; ti->discards_supported = 1; It looks like setting above 2 parameters did not get me any REQ_DISCARD requests when "fstrim" utility is used. The operations fails with "Operation not supported" from kernel funcion: blkdev_issue_discard because max_discard_sectors for device queue is set to 0. max_discard_sectors = min(q->limits.max_discard_sectors, UINT_MAX >> 9); if (unlikely(!max_discard_sectors)) return -EOPNOTSUPP; When I further digged into the device mapper code, I found that dm_calculate_queue_limits function sets the queue limits to default values of 0 for both limits as well as ti_limits and thus blk_stack_limits eventually sets max_discard_sectors to 0 for the device mapper target. So, if my device mapper target does not implement iterate_devices function (used to set max_discard_sectors based on underlying physical device's discard support) or underlying physical device does not support discards, then there is no way for device mapper target to set max_discard_sectors more than 0 to receive REQ_DISCARD commands. Is this analysis correct? Or I am missing something? Appreciate the help / advice. Thanks and Regards, Amar