Trim/Discard are two names for the same feature: a request to a storage system, telling it that some data range is being used anymore and can get recycled. This call originates in Flash-based storages (SSDs, FusionIO cards, etc.), which cannot easily rewrite a sector but instead have to erase and write the (new) data again (incurring some latency cost). For more details, see eg. the [[https://en.wikipedia.org/wiki/Trim_%28computing%29,wikipedia page]].
Since 8.4.3 DRBD includes support for Trim/Discard. You don’t need to configure or enable anything; if DRBD detects that the local (underlying) storage system allows using these commands, it will transparently enable them and pass such requests through.
The effect is that eg. a recent-enough
mkfs.ext4 on a multi-TB volume can
shorten the initial sync time to a few seconds to minutes - just by telling
DRBD (which will relay that information to all connected nodes) that most/all
of the storage is now to be seen as invalidated.
Nodes that connect to that resource later on will not have seen the
Trim/Discard requests, and will therefore start a full resync; depending on
kernel version and file system a call to
fstrim might give the wanted result,
even if you don’t have storage with Trim/Discard support, some virtual block devices will provide you with the same feature, for example Thin LVM.
 For example, a deleted file’s data.