On Sun, Mar 25 2012 at 6:46pm -0400, Amar Mudrankit <amar.mudrankit@xxxxxxxxx> wrote: > 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. I'm just curious but: what will your target do in general? > In my target, I set: > > ti->num_discard_requests = 1; > ti->discards_supported = 1; You'll also want to set ti->discards_supported see: http://git.kernel.org/linus/4c259327 > 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? All DM targets should implement .iterate_devices -- intent is to iterate over all data devices. You'll also want to implement .io_hints to set limits->max_discard_sectors (like drivers/md/dm-thin.c does). Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
