Discard,Trim,Unmap

Description

Discard or Trim or Unmap is the technology to let know underlying storage about deleted/unused block. It is important for SSD drive to achieve their good condition or for thin provisioned storages for saving storage pool space.

TRIM/UNMAP capability

lsblk -Dshould shows discard/trim capabilities, but it is no true at all. It actual show "discard_zeroes_data". It is only part of discarding. No all device has to support "read return zero".

Fstrim for example get trim capabilities from /sys/block/<disk>/queue/discard_granularity (0 means not supported).

This oneliner print discard_granularity for all devices:

for i in `ls -1 /sys/block/`; do echo -n $i": "; cat /sys/block/$i/queue/discard_granularity; done