Add snapdev=[hidden|visible] dataset property

The new snapdev dataset property may be set to control the
visibility of zvol snapshot devices.  By default this value
is set to 'hidden' which will prevent zvol snapshots from
appearing under /dev/zvol/ and /dev/<dataset>/.  When set to
'visible' all zvol snapshots for the dataset will be visible.

This functionality was largely added because when automatic
snapshoting is enabled large numbers of read-only zvol snapshots
will be created.  When creating these devices the kernel will
attempt to read their partition tables, and blkid will attempt
to identify any filesystems on those partitions.  This leads
to a variety of issues:

1) The zvol partition tables will be read in the context of
   the `modprobe zfs` for automatically imported pools.  This
   is undesirable and should be done asynchronously, but for
   now reducing the number of visible devices helps.

2) Udev expects to be able to complete its work for a new
   block devices fairly quickly.  When many zvol devices are
   added at the same time this is no longer be true.  It can
   lead to udev timeouts and missing /dev/zvol links.

3) Simply having lots of devices in /dev/ can be aukward from
   a management standpoint.  Hidding the devices your unlikely
   to ever use helps with this.  Any snapshot device which is
   needed can be made visible by changing the snapdev property.

NOTE: This patch changes the default behavior for zvols which
      was effectively 'snapdev=visible'.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1235
Closes #945
Issue #956
Issue #756

include/sys/fs/zfs.h

@@ -128,6 +128,7 @@ typedef enum {
 	ZFS_PROP_REFRATIO,
 	ZFS_PROP_WRITTEN,
 	ZFS_PROP_CLONES,
+	ZFS_PROP_SNAPDEV,
 	ZFS_NUM_PROPS
 } zfs_prop_t;
 

include/sys/zfs_ioctl.h

@@ -46,6 +46,12 @@ extern "C" {
 #define	ZFS_SNAPDIR_HIDDEN		0
 #define	ZFS_SNAPDIR_VISIBLE		1
 
+/*
+ * Property values for snapdev
+ */
+#define	ZFS_SNAPDEV_HIDDEN		0
+#define	ZFS_SNAPDEV_VISIBLE		1
+
 /*
  * Field manipulation macros for the drr_versioninfo field of the
  * send stream header.

include/sys/zvol.h

@@ -44,6 +44,7 @@ extern int zvol_remove_minor(const char *);
 extern void zvol_remove_minors(const char *);
 extern int zvol_set_volsize(const char *, uint64_t);
 extern int zvol_set_volblocksize(const char *, uint64_t);
+extern int zvol_set_snapdev(const char *, uint64_t);
 
 extern int zvol_init(void);
 extern void zvol_fini(void);

lib/libzfs/libzfs_dataset.c

@@ -4016,6 +4016,14 @@ zvol_create_link_common(libzfs_handle_t *hdl, const char *dataset, int ifexists)
 			 */
 			return (0);
 
+		case ENODEV:
+			/*
+			 * snapdev set to hidden :
+			 *  device creation was not permitted (see zvol.c)
+			 *  ignore error quietly
+			 */
+			return (0);
+
 		case ENOENT:
 			/*
 			 * Dataset does not exist in the kernel.  If we

man/man8/zfs.8

@@ -1081,6 +1081,17 @@ When the \fBsharenfs\fR property is changed for a dataset, the dataset and any c
 Provide a hint to ZFS about handling of synchronous requests in this dataset. If \fBlogbias\fR is set to \fBlatency\fR (the default), ZFS will use pool log devices (if configured) to handle the requests at low latency. If \fBlogbias\fR is set to \fBthroughput\fR, ZFS will not use configured pool log devices. ZFS will instead optimize synchronous operations for global pool throughput and efficient use of resources.
 .RE
 
+.sp
+.ne 2
+.mk
+.na
+\fB\fBsnapdev\fR=\fBhidden\fR | \fBvisible\fR\fR
+.ad
+.sp .6
+.RS 4n
+Controls whether the snapshots devices of zvol's are hidden or visible. The default value is \fBhidden\fR.
+.RE
+
 .sp
 .ne 2
 .mk

module/zcommon/zfs_prop.c

@@ -106,6 +106,12 @@ zfs_prop_init(void)
 		{ NULL }
 	};
 
+	static zprop_index_t snapdev_table[] = {
+		{ "hidden",	ZFS_SNAPDEV_HIDDEN },
+		{ "visible",	ZFS_SNAPDEV_VISIBLE },
+		{ NULL }
+	};
+
 	static zprop_index_t acl_inherit_table[] = {
 		{ "discard",	ZFS_ACL_DISCARD },
 		{ "noallow",	ZFS_ACL_NOALLOW },
@@ -217,6 +223,9 @@ zfs_prop_init(void)
 	zprop_register_index(ZFS_PROP_SNAPDIR, "snapdir", ZFS_SNAPDIR_HIDDEN,
 	    PROP_INHERIT, ZFS_TYPE_FILESYSTEM,
 	    "hidden | visible", "SNAPDIR", snapdir_table);
+	zprop_register_index(ZFS_PROP_SNAPDEV, "snapdev", ZFS_SNAPDEV_HIDDEN,
+	    PROP_INHERIT, ZFS_TYPE_FILESYSTEM | ZFS_TYPE_VOLUME,
+	    "hidden | visible", "SNAPDEV", snapdev_table);
 	zprop_register_index(ZFS_PROP_ACLINHERIT, "aclinherit",
 	    ZFS_ACL_RESTRICTED, PROP_INHERIT, ZFS_TYPE_FILESYSTEM,
 	    "discard | noallow | restricted | passthrough | passthrough-x",

module/zfs/zfs_ioctl.c

@@ -2184,6 +2184,9 @@ zfs_prop_set_special(const char *dsname, zprop_source_t source,
 	case ZFS_PROP_VOLSIZE:
 		err = zvol_set_volsize(dsname, intval);
 		break;
+	case ZFS_PROP_SNAPDEV:
+		err = zvol_set_snapdev(dsname, intval);
+		break;
 	case ZFS_PROP_VERSION:
 	{
 		zfs_sb_t *zsb;

module/zfs/zvol.c

@@ -1288,7 +1288,28 @@ zvol_free(zvol_state_t *zv)
 }
 
 static int
-__zvol_create_minor(const char *name)
+__zvol_snapdev_hidden(const char *name)
+{
+        uint64_t snapdev;
+        char *parent;
+        char *atp;
+        int error = 0;
+
+        parent = kmem_alloc(MAXPATHLEN, KM_SLEEP);
+        (void) strlcpy(parent, name, MAXPATHLEN);
+
+        if ((atp = strrchr(parent, '@')) != NULL) {
+                *atp = '\0';
+                error = dsl_prop_get_integer(parent, "snapdev", &snapdev, NULL);
+                if ((error == 0) && (snapdev == ZFS_SNAPDEV_HIDDEN))
+                        error = ENODEV;
+        }
+        kmem_free(parent, MAXPATHLEN);
+        return (error);
+}
+
+static int
+__zvol_create_minor(const char *name, boolean_t ignore_snapdev)
 {
 	zvol_state_t *zv;
 	objset_t *os;
@@ -1305,6 +1326,12 @@ __zvol_create_minor(const char *name)
 		goto out;
 	}
 
+	if (ignore_snapdev == B_FALSE) {
+		error = __zvol_snapdev_hidden(name);
+		if (error)
+			goto out;
+	}
+
 	doi = kmem_alloc(sizeof(dmu_object_info_t), KM_SLEEP);
 
 	error = dmu_objset_own(name, DMU_OST_ZVOL, B_TRUE, zvol_tag, &os);
@@ -1386,7 +1413,7 @@ zvol_create_minor(const char *name)
 	int error;
 
 	mutex_enter(&zvol_state_lock);
-	error = __zvol_create_minor(name);
+	error = __zvol_create_minor(name, B_FALSE);
 	mutex_exit(&zvol_state_lock);
 
 	return (error);
@@ -1434,7 +1461,7 @@ zvol_create_minors_cb(spa_t *spa, uint64_t dsobj,
 	if (strchr(dsname, '/') == NULL)
 		return 0;
 
-	(void) __zvol_create_minor(dsname);
+	(void) __zvol_create_minor(dsname, B_FALSE);
 	return (0);
 }
 
@@ -1502,6 +1529,35 @@ zvol_remove_minors(const char *pool)
 	kmem_free(str, MAXNAMELEN);
 }
 
+static int
+snapdev_snapshot_changed_cb(const char *dsname, void *arg) {
+	uint64_t snapdev = *(uint64_t *) arg;
+
+	if (strchr(dsname, '@') == NULL)
+		return 0;
+
+	switch (snapdev) {
+		case ZFS_SNAPDEV_VISIBLE:
+			mutex_enter(&zvol_state_lock);
+			(void) __zvol_create_minor(dsname, B_TRUE);
+			mutex_exit(&zvol_state_lock);
+			break;
+		case ZFS_SNAPDEV_HIDDEN:
+			(void) zvol_remove_minor(dsname);
+			break;
+	}
+	return 0;
+}
+
+int
+zvol_set_snapdev(const char *dsname, uint64_t snapdev) {
+	(void) dmu_objset_find((char *) dsname, snapdev_snapshot_changed_cb,
+		&snapdev, DS_FIND_SNAPSHOTS | DS_FIND_CHILDREN);
+	/* caller should continue to modify snapdev property */
+	return (-1);
+}
+
+
 int
 zvol_init(void)
 {

scripts/zconfig.sh

@@ -192,6 +192,7 @@ test_3() {
 	${ZFS_SH} zfs="spa_config_path=${TMP_CACHE}" || fail 1
 	${ZPOOL_CREATE_SH} -p ${POOL_NAME} -c lo-raidz2 || fail 2
 	${ZFS} create -V 100M ${FULL_ZVOL_NAME} || fail 3
+	${ZFS} set snapdev=visible ${FULL_ZVOL_NAME} || fail 3
 	label /dev/zvol/${FULL_ZVOL_NAME} msdos || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME} primary 1% 50% || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME} primary 51% -1 || fail 4
@@ -245,6 +246,7 @@ test_4() {
 	${ZFS_SH} zfs="spa_config_path=${TMP_CACHE}" || fail 1
 	${ZPOOL_CREATE_SH} -p ${POOL_NAME} -c lo-raidz2 || fail 2
 	${ZFS} create -V 100M ${FULL_ZVOL_NAME} || fail 3
+	${ZFS} set snapdev=visible ${FULL_ZVOL_NAME} || fail 3
 	label /dev/zvol/${FULL_ZVOL_NAME} msdos || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME} primary 1% 50% || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME} primary 51% -1 || fail 4
@@ -335,6 +337,7 @@ test_6() {
 	${ZFS_SH} zfs="spa_config_path=${TMP_CACHE}" || fail 1
 	${ZPOOL_CREATE_SH} -p ${POOL_NAME} -c lo-raid0 || fail 2
 	${ZFS} create -V 800M ${FULL_ZVOL_NAME} || fail 3
+	${ZFS} set snapdev=visible ${FULL_ZVOL_NAME} || fail 3
 	label /dev/zvol/${FULL_ZVOL_NAME} msdos || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME} primary 1 -1 || fail 4
 	format /dev/zvol/${FULL_ZVOL_NAME}-part1 ext2 || fail 5
@@ -394,6 +397,7 @@ test_7() {
 	${ZFS_SH} zfs="spa_config_path=${TMP_CACHE}" || fail 1
 	${ZPOOL_CREATE_SH} -p ${POOL_NAME} -c lo-raidz2 || fail 2
 	${ZFS} create -V 300M ${FULL_ZVOL_NAME} || fail 3
+	${ZFS} set snapdev=visible ${FULL_ZVOL_NAME} || fail 3
 	label /dev/zvol/${FULL_ZVOL_NAME} msdos || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME} primary 1 -1 || fail 4
 	format /dev/zvol/${FULL_ZVOL_NAME}-part1 ext2 || fail 5
@@ -481,6 +485,7 @@ test_8() {
 	${ZPOOL_CREATE_SH} -p ${POOL_NAME1} -c lo-raidz2 || fail 2
 	${ZPOOL_CREATE_SH} -p ${POOL_NAME2} -c lo-raidz2 || fail 2
 	${ZFS} create -V 300M ${FULL_ZVOL_NAME1} || fail 3
+	${ZFS} set snapdev=visible ${FULL_ZVOL_NAME1} || fail 3
 	label /dev/zvol/${FULL_ZVOL_NAME1} msdos || fail 4
 	partition /dev/zvol/${FULL_ZVOL_NAME1} primary 1 -1 || fail 4
 	format /dev/zvol/${FULL_ZVOL_NAME1}-part1 ext2 || fail 5