MFC r260150: MFV r259170:

4370 avoid transmitting holes during zfs send

4371 DMU code clean up

illumos/illumos-gate@43466aae47bfcd2ad9bf501faec8e75c08095e4f

NOTE: Make sure the boot code is updated if a zpool upgrade is
done on boot zpool.

1 files changed, 71 insertions, 1 deletions

diff --git a/cddl/contrib/opensolaris/cmd/zpool/zpool-features.7 b/cddl/contrib/opensolaris/cmd/zpool/zpool-features.7
index d807257..43eede5 100644
--- a/cddl/contrib/opensolaris/cmd/zpool/zpool-features.7
+++ b/cddl/contrib/opensolaris/cmd/zpool/zpool-features.7
@@ -23,7 +23,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd October 08, 2013
+.Dd December 31, 2013
 .Dt ZPOOL-FEATURES 7
 .Os
 .Sh NAME
@@ -286,6 +286,76 @@ and will be returned to the
 .Sy enabled
 state when all datasets that use
 this feature are destroyed.
+.It Sy enabled_txg
+.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:enabled_txg"
+.It GUID Ta com.delphix:enabled_txg
+.It READ\-ONLY COMPATIBLE Ta yes
+.It DEPENDENCIES Ta none
+.El
+.Pp
+Once this feature is enabled ZFS records the transaction group number
+in which new features are enabled. This has no user-visible impact,
+but other features may depend on this feature.
+.Pp
+This feature becomes
+.Sy active
+as soon as it is enabled and will
+never return to being
+.Sy enabled .
+.It Sy hole_birth
+.Bl -column "READ\-ONLY COMPATIBLE" "com.delphix:hole_birth"
+.It GUID Ta com.delphix:hole_birth
+.It READ\-ONLY COMPATIBLE Ta no
+.It DEPENDENCIES Ta enabled_txg
+.El
+.Pp
+This feature improves performance of incremental sends
+.Pq Dq zfs send -i
+and receives for objects with many holes.
+The most common case of
+hole-filled objects is zvols.
+.Pp
+An incremental send stream from snapshot
+.Sy A
+to snapshot
+.Sy B
+contains information about every block that changed between
+.Sy A
+and
+.Sy B .
+Blocks which did not change between those snapshots can be
+identified and omitted from the stream using a piece of metadata called
+the 'block birth time', but birth times are not recorded for holes
+.Pq blocks filled only with zeroes .
+Since holes created after
+.Sy A
+cannot be
+distinguished from holes created before
+.Sy A ,
+information about every
+hole in the entire filesystem or zvol is included in the send stream.
+.Pp
+For workloads where holes are rare this is not a problem.
+However, when
+incrementally replicating filesystems or zvols with many holes
+.Pq for example a zvol formatted with another filesystem
+a lot of time will
+be spent sending and receiving unnecessary information about holes that
+already exist on the receiving side.
+.Pp
+Once the
+.Sy hole_birth
+feature has been enabled the block birth times
+of all new holes will be recorded.
+Incremental sends between snapshots
+created after this feature is enabled will use this new metadata to avoid
+sending information about holes that already exist on the receiving side.
+.Pp
+This feature becomes
+.Sy active
+as soon as it is enabled and will
+never return to being
+.Sy enabled .
 .El
 .Sh SEE ALSO
 .Xr zpool 8