Tool for discarding block device ranges containing only zeroes
This Linux utitity scans a block device for ranges of blocks containing
only zeroes, and if it finds any, issues a BLKDISCARD ioctl to
get rid of them.
It is similar to the fstrim tool for file systems. The intended use of discard0 is for block devices that are not used as file systems directly (e.g. raw devices used by virtual machines). If in doubt, use fstrim!
THIS TOOL MAY DESTROY YOUR DATA!
Some block devices consistently return zeroes on discarded blocks. Using this utility is only safe on devices which consistently return zeroes on discarded blocks.
I wrote this utility for discarding zero-blocks on a thin volumes. It should be safe for LVM thin, because the LVM thin code consistently returns zeroes for unmapped blocks. It worked well in my tests.
But there is NO GUARANTEE. Backup your data before using this. YOU HAVE BEEN WARNED!
Q: Why does the tool use BLKDISCARD and not fallocate(FALLOC_FL_PUNCH_HOLE)?
A: It seems that currently (mid 2018), not all devices that return
zeroes consistently after a discard report this correctly to the kernel (sysfs
attribute queue/write_zeroes_max_bytes). In particular, dm-thin doesn't
advertize support for this feature, although it quite obviously has
it. Therefore fallocate(FALLOC_FL_PUNCH_HOLE) wouldn't work on LVM thin
provisioning devices.