Btrfs Ready For Linux 5.6 With Async Discard For Better Efficiency + Performance

Btrfs in the now-stable Linux 5.5 kernel is exciting for its new RAID1C3/RAID1C4 capability allowing three/four copies of data rather than just two while looking ahead to Linux 5.6 is further feature work on this Linux file-system.

The big Btrfs item on the table with Linux 5.6 is async discard support developed by Facebook engineers and so solid-state drive TRIM/DISCARD support isn't done synchronously. This led to an efficiency improvement and lower read latencies when deploying the feature on Facebook servers.

The PR explains the async discard details:

* "mount -o discard=async" to enable it
* freed extents are not discarded immediatelly, but grouped together and trimmed later, with IO rate limiting
* the "sync" mode submits short extents that could have been ignored completely by the device, for SATA prior to 3.1 the requests are unqueued and have a big impact on performance
* the actual discard IO requests have been moved out of transaction commit to a worker thread, improving commit latency
* IO rate and request size can be tuned by sysfs files, for now enabled only with CONFIG_BTRFS_DEBUG as we might need to add/delete the files and don't have a stable-ish ABI for general use, defaults are conservative

The Btrfs for Linux 5.6 pull also now exports more device state information via sysfs, a variety of bug fixes, dropping of snapshot aware code that has been disabled for years due to performance issues, and other code improvements.