illumos gate

Originally filed by Matt Ahrens:

ZFS should allow highly-compressible blocks to have their compressed data
stored in the block pointer itself.  This provides a space savings and
performance improvement (not having to write/read these blocks).

The benefit should be seen when an application creates highly compressible blocks (e.g. some applications write block "headers" to every file system block, but leave the rest of the block untouched, if the header can be compressed to less than the size of a bp this feature can be used). It should also help compress some of ZFS's internal metadata.