One of these new 'Feature Flags' is support for LZ4 Compression. ZFS has long supported transparent compression of datasets (data is automatically compressed as it is written) with a number of algorithms: lzjb, gzip [1-9] and zle. Of the available algorithms, lzjb was the most popular because of its lower CPU consumption, however specific datasets could be compressed with various levels of gzip to gain additional space savings at the cost of more CPU usage.
LZ4 is a new BSD licensed high performance multi-core scalable compression algorithm. In addition to better compression in less time, it also features extremely fast decompression rates. Compared to the default LZJB compression algorithm used by ZFS, LZ4 is 50% faster when compressing compressible data and over three times faster when attempting to compress incompressible data. The performance on incompressible data is a large improvement, this comes from an 'early abort' feature, if ZFS detects that the compression savings is less than 12.5% then compression is aborted and the block is written uncompressed (especially useful for large multimedia files that are already compressed). In addition, decompression is approximately 80% faster; on a modern CPU LZ4 is capable of compression at 500 MB/s and decompression at 1500 MB/s per CPU core. These numbers mean that for some workloads, compression will actually give increased performance, even with the CPU usage penalty, because data can be read from the disks at the same speed as uncompressed data but then once decompressed provides a much higher effective throughput. This also means it is now possible to use dataset compression on file systems that are storing databases, without a heavy latency penalty. LZ4 decompression at 1.5 GB/s on 8k blocks means the additional latency is only 5 microseconds, which is an order of magnitude faster than even the fasted SSDs currently available.
In the end, the gain you get from switching to LZ4 for compression on your dataset will depend on how compressible the data you are writing is.
To enable LZ4 compression on a dataset:
# zfs set compression=lz4 poolname/dataset
In order to make use of LZ4 your pool will need to be upgraded to v5000 (Note: this means that your pool will only be readable by FreeBSD 8.4 and 9.2 or later).