DISCARD support in kernel driver

[Jean-Tiare LE BIGOT <jean-tiare.le-bigot@ovh.net>]

Hi,

I started to implement 'DISCARD' support in RBD kernel driver as 
described on http://tracker.ceph.com/issues/190

This first (easy) step was to add at the end of 
drivers/block/rbd.c:rbd_init_disk

     /* Advertise discard support for aligned blocks */
     queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
     disk->queue->limits.discard_granularity = segment_size;
     disk->queue->limits.discard_alignment = segment_size;

With this both 'mount -o discard' and 'fstrim' stopped to complain about 
missing 'DISCARD' support. Good !

Next step was to add support on `rbd_request_fn` which I did (sort of). 
But here I'm stuck. If I understood well FS drivers uses `REQ_DISCARD` 
flag of `rq->cmd_flags` to notify the driver of the discard operation. 
But the problem is that (rq->cmd_flags & REQ_DISCARD) never appears to 
be set.

I tried copying then removing large files, lots of small files, forcing 
fstrim. But I never got it. I must be missing something obvious but I 
can't manage to find what ? Do you have any clue what could be wrong ?

Thanks,

-- 
Jean-Tiare, shared-hosting team