Metztli Reiser4 added support for 'discard' mount option for Reiser4 in Debian Installer (d-i) netboot USB/ISO image...
because:
"Yep, reiser4 supports both, fstrim and discard [mount option for SSDs, SMR hard disks or thin provisioned storage]. So, there is a support (implemented ioctl) for fstrim(8) utility and a resp. mount option for discard. Moreover, reiser4 performs 'precise' discard. It means that once the discard is turned on on the mounted volume, the stuff (a set of on-disk units) which is a subject for discard, does not accumulate in time. So that fstrim becomes a kind of 'secondary option' which exists just to discard the stuff accumulated on disk for some 'curious' reasons, typically when the partition was formatted without -d option, or user forgot to mount the volume with 'discard' mount option."




WARNING: discard atom failed (-95)
If operating in a virtual environment, such as that of VirtualBox, the dmesg command utility will output similar as:
reiser4[wireplumber(601)]: reiser4_post_write_back_hook (fs/reiser4/block_alloc.c:1154)[intelfx-8]:
WARNING: discard atom failed (-95)
It means that your [virtual device slice] doesn't support TRIM command.