Make createtxg and guid properties public

Document the existence of `createtxg` and `guid` native properties
in man pages and zfs command output.

One of the great features of ZFS is incremental replication of
snapshots, possibly between pools on different machines.

Shell scripts are commonly used to auomate this procedure. They have to
find the most recent common snapshot between both sides and then
perform incremental send & recv.
Currently, scripts rely on the sorting order of `zfs list`, which
defaults to `createtxg`, and the assumption that snapshot names on
either side do not change.

By making `createtxg` and `guid` part of the public ZFS interface,
scripts are enabled to use

  a) `createtxg` to determine the logical & temporal order of snapshots
     (the creation property is not an equivalent substitute since
      multiple snapshots may be created within one second)
  b) `guid` to uniquely identify a snapshot, independent of its current
      display name

This has the potential of making scripts safer and correct.

Reviewed-by: George Melikov <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: DHE <[email protected]>
Reviewed-by: Richard Laager <[email protected]>
Signed-off-by: Christian Schwarz <[email protected]>
Closes #6102 

4 files changed, 30 insertions, 5 deletions

diff --git a/include/sys/fs/zfs.h b/include/sys/fs/zfs.h
index ba6487612..5b7bbb689 100644
--- a/include/sys/fs/zfs.h
+++ b/include/sys/fs/zfs.h
@@ -113,7 +113,7 @@ typedef enum {
 	ZFS_PROP_SNAPDIR,
 	ZFS_PROP_PRIVATE,		/* not exposed to user, temporary */
 	ZFS_PROP_ACLINHERIT,
-	ZFS_PROP_CREATETXG,		/* not exposed to the user */
+	ZFS_PROP_CREATETXG,
 	ZFS_PROP_NAME,			/* not exposed to the user */
 	ZFS_PROP_CANMOUNT,
 	ZFS_PROP_ISCSIOPTIONS,		/* not exposed to the user */
diff --git a/lib/libzfs/libzfs_dataset.c b/lib/libzfs/libzfs_dataset.c
index 0f18fd690..f1346b69c 100644
--- a/lib/libzfs/libzfs_dataset.c
+++ b/lib/libzfs/libzfs_dataset.c
@@ -2695,6 +2695,7 @@ zfs_prop_get(zfs_handle_t *zhp, zfs_prop_t prop, char *propbuf, size_t proplen,
 		break;
 
 	case ZFS_PROP_GUID:
+	case ZFS_PROP_CREATETXG:
 		/*
 		 * GUIDs are stored as numbers, but they are identifiers.
 		 * We don't want them to be pretty printed, because pretty
diff --git a/man/man8/zfs.8 b/man/man8/zfs.8
index 672079637..5d0b79b99 100644
--- a/man/man8/zfs.8
+++ b/man/man8/zfs.8
@@ -441,6 +441,18 @@ For non-snapshots, the compression ratio achieved for the \fBused\fR space of th
 .sp
 .ne 2
 .na
+\fB\fBcreatetxg\fR
+.ad
+.sp .6
+.RS 4n
+The transaction group (TXG) in which the dataset was created. Bookmarks have the same \fBcreatetxg\fR as the snapshot they are initially tied to.
+.sp
+\fBcreatetxg\fR is suitable for ordering a list of snapshots, e.g. for incremental \fBsend\fR & \fBrecv\fR.
+.RE
+
+.sp
+.ne 2
+.na
 \fB\fBcreation\fR\fR
 .ad
 .sp .6
@@ -488,6 +500,18 @@ been set somewhere in the tree under which the dataset resides.
 .sp
 .ne 2
 .na
+\fB\fBguid\fR
+.ad
+.sp .6
+.RS 4n
+The 64 bit GUID of this dataset or bookmark, which does not change over its entire lifetime.
+.sp
+When a snapshot is sent to another pool, the received snapshot has the same GUID. Thus, \fBguid\fR is suitable to identify a snapshot across pools.
+.RE
+
+.sp
+.ne 2
+.na
 \fB\fBlogicalreferenced\fR\fR
 .ad
 .sp .6
diff --git a/module/zcommon/zfs_prop.c b/module/zcommon/zfs_prop.c
index 82641b490..97736ee27 100644
--- a/module/zcommon/zfs_prop.c
+++ b/module/zcommon/zfs_prop.c
@@ -439,6 +439,10 @@ zfs_prop_init(void)
 	zprop_register_number(ZFS_PROP_SNAPSHOT_COUNT, "snapshot_count",
 	    UINT64_MAX, PROP_READONLY, ZFS_TYPE_FILESYSTEM | ZFS_TYPE_VOLUME,
 	    "<count>", "SSCOUNT");
+	zprop_register_number(ZFS_PROP_GUID, "guid", 0, PROP_READONLY,
+	    ZFS_TYPE_DATASET | ZFS_TYPE_BOOKMARK, "<uint64>", "GUID");
+	zprop_register_number(ZFS_PROP_CREATETXG, "createtxg", 0, PROP_READONLY,
+	    ZFS_TYPE_DATASET | ZFS_TYPE_BOOKMARK, "<uint64>", "CREATETXG");
 
 	/* default number properties */
 	zprop_register_number(ZFS_PROP_QUOTA, "quota", 0, PROP_DEFAULT,
@@ -466,8 +470,6 @@ zfs_prop_init(void)
 	    ZFS_TYPE_FILESYSTEM, "512 to 1M, power of 2", "RECSIZE");
 
 	/* hidden properties */
-	zprop_register_hidden(ZFS_PROP_CREATETXG, "createtxg", PROP_TYPE_NUMBER,
-	    PROP_READONLY, ZFS_TYPE_DATASET | ZFS_TYPE_BOOKMARK, "CREATETXG");
 	zprop_register_hidden(ZFS_PROP_NUMCLONES, "numclones", PROP_TYPE_NUMBER,
 	    PROP_READONLY, ZFS_TYPE_SNAPSHOT, "NUMCLONES");
 	zprop_register_hidden(ZFS_PROP_NAME, "name", PROP_TYPE_STRING,
@@ -477,8 +479,6 @@ zfs_prop_init(void)
 	zprop_register_hidden(ZFS_PROP_STMF_SHAREINFO, "stmf_sbd_lu",
 	    PROP_TYPE_STRING, PROP_INHERIT, ZFS_TYPE_VOLUME,
 	    "STMF_SBD_LU");
-	zprop_register_hidden(ZFS_PROP_GUID, "guid", PROP_TYPE_NUMBER,
-	    PROP_READONLY, ZFS_TYPE_DATASET | ZFS_TYPE_BOOKMARK, "GUID");
 	zprop_register_hidden(ZFS_PROP_USERACCOUNTING, "useraccounting",
 	    PROP_TYPE_NUMBER, PROP_READONLY, ZFS_TYPE_DATASET,
 	    "USERACCOUNTING");