Performance evaluation of Linux Discard Support
Discard tells a device which blocks it no longer has to keep. Benchmarks of what issuing it actually costs, on real SSDs and on thinly provisioned storage.
This is the talk where I presented the discard numbers. How discard works down
in the block layer, how it reaches the device through the BLKDISCARD ioctl,
and what the measurements showed once I ran them against real drives rather than
reasoning about the specification.
Discard helps. It keeps an SSD from degrading as it fills and ages, and it can
speed up file system operations as well. What it costs you depends entirely on
how you issue it, and that varies between drives far more than anyone expected.
Which is why ext4 and btrfs had grown periodic discard support by then, and why
fstrim exists: batch the free space up and discard it on a mounted file system
when it suits you, instead of on every delete.
The benchmark numbers are in the slides.