[dm-devel] Query about dm device max_discard_sectors

[Amar Mudrankit <amar mudrankit gmail com>]

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