Illumos 4757, 4913

4757 ZFS embedded-data block pointers ("zero block compression")
4913 zfs release should not be subject to space checks

Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Max Grossman <max.grossman@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/4757
  https://www.illumos.org/issues/4913
  https://github.com/illumos/illumos-gate/commit/5d7b4d4

Porting notes:

For compatibility with the fastpath code the zio_done() function
needed to be updated.  Because embedded-data block pointers do
not require DVAs to be allocated the associated vdevs will not
be marked and therefore should not be unmarked.

Ported by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2544

man/man5/zpool-features.5

@@ -358,6 +358,33 @@ never return to being \fBenabled\fB.
 
 .RE
 
+.sp
+.ne 2
+.na
+\fB\fBembedded_data\fR\fR
+.ad
+.RS 4n
+.TS
+l l .
+GUID	com.delphix:embedded_data
+READ\-ONLY COMPATIBLE	no
+DEPENDENCIES	none
+.TE
+
+This feature improves the performance and compression ratio of
+highly-compressible blocks.  Blocks whose contents can compress to 112 bytes
+or smaller can take advantage of this feature.
+
+When this feature is enabled, the contents of highly-compressible blocks are
+stored in the block "pointer" itself (a misnomer in this case, as it contains
+the compresseed data, rather than a pointer to its location on disk).  Thus
+the space of the block (one sector, typically 512 bytes or 4KB) is saved,
+and no additional i/o is needed to read and write the data block.
+
+This feature becomes \fBactive\fR as soon as it is enabled and will
+never return to being \fBenabled\fR.
+
+.RE
 
 .SH "SEE ALSO"
 \fBzpool\fR(8)