4 * The contents of this file are subject to the terms of the
5 * Common Development and Distribution License (the "License").
6 * You may not use this file except in compliance with the License.
8 * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
9 * or http://www.opensolaris.org/os/licensing.
10 * See the License for the specific language governing permissions
11 * and limitations under the License.
13 * When distributing Covered Code, include this CDDL HEADER in each
14 * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
15 * If applicable, add the following below this CDDL HEADER, with the
16 * fields enclosed by brackets "[]" replaced with your own identifying
17 * information: Portions Copyright [yyyy] [name of copyright owner]
23 * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
24 * Copyright (C) 2011 Lawrence Livermore National Security, LLC.
25 * Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
27 * Rewritten for Linux by:
28 * Rohan Puri <rohan.puri15@gmail.com>
29 * Brian Behlendorf <behlendorf1@llnl.gov>
33 * ZFS control directory (a.k.a. ".zfs")
35 * This directory provides a common location for all ZFS meta-objects.
36 * Currently, this is only the 'snapshot' and 'shares' directory, but this may
37 * expand in the future. The elements are built dynamically, as the hierarchy
38 * does not actually exist on disk.
40 * For 'snapshot', we don't want to have all snapshots always mounted, because
41 * this would take up a huge amount of space in /etc/mnttab. We have three
44 * ctldir ------> snapshotdir -------> snapshot
50 * The 'snapshot' node contains just enough information to lookup '..' and act
51 * as a mountpoint for the snapshot. Whenever we lookup a specific snapshot, we
52 * perform an automount of the underlying filesystem and return the
53 * corresponding inode.
55 * All mounts are handled automatically by an user mode helper which invokes
56 * the mount mount procedure. Unmounts are handled by allowing the mount
57 * point to expire so the kernel may automatically unmount it.
59 * The '.zfs', '.zfs/snapshot', and all directories created under
60 * '.zfs/snapshot' (ie: '.zfs/snapshot/<snapname>') all share the same
61 * share the same zfs_sb_t as the head filesystem (what '.zfs' lives under).
63 * File systems mounted on top of the '.zfs/snapshot/<snapname>' paths
64 * (ie: snapshots) are complete ZFS filesystems and have their own unique
65 * zfs_sb_t. However, the fsid reported by these mounts will be the same
66 * as that used by the parent zfs_sb_t to make NFS happy.
69 #include <sys/types.h>
70 #include <sys/param.h>
72 #include <sys/systm.h>
73 #include <sys/sysmacros.h>
74 #include <sys/pathname.h>
76 #include <sys/vfs_opreg.h>
77 #include <sys/zfs_ctldir.h>
78 #include <sys/zfs_ioctl.h>
79 #include <sys/zfs_vfsops.h>
80 #include <sys/zfs_vnops.h>
83 #include <sys/dsl_deleg.h>
84 #include <sys/mount.h>
86 #include "zfs_namecheck.h"
89 * Control Directory Tunables (.zfs)
91 int zfs_expire_snapshot = ZFSCTL_EXPIRE_SNAPSHOT;
93 static zfs_snapentry_t *
94 zfsctl_sep_alloc(void)
96 return kmem_zalloc(sizeof (zfs_snapentry_t), KM_SLEEP);
100 zfsctl_sep_free(zfs_snapentry_t *sep)
102 kmem_free(sep->se_name, MAXNAMELEN);
103 kmem_free(sep->se_path, PATH_MAX);
104 kmem_free(sep, sizeof (zfs_snapentry_t));
108 * Attempt to expire an automounted snapshot, unmounts are attempted every
109 * 'zfs_expire_snapshot' seconds until they succeed. The work request is
110 * responsible for rescheduling itself and freeing the zfs_expire_snapshot_t.
113 zfsctl_expire_snapshot(void *data)
115 zfs_snapentry_t *sep;
119 sep = spl_get_work_data(data, zfs_snapentry_t, se_work.work);
120 zsb = ITOZSB(sep->se_inode);
122 error = zfsctl_unmount_snapshot(zsb, sep->se_name, MNT_EXPIRE);
124 schedule_delayed_work(&sep->se_work, zfs_expire_snapshot * HZ);
128 snapentry_compare(const void *a, const void *b)
130 const zfs_snapentry_t *sa = a;
131 const zfs_snapentry_t *sb = b;
132 int ret = strcmp(sa->se_name, sb->se_name);
143 zfsctl_is_node(struct inode *ip)
145 return (ITOZ(ip)->z_is_ctldir);
149 zfsctl_is_snapdir(struct inode *ip)
151 return (zfsctl_is_node(ip) && (ip->i_ino <= ZFSCTL_INO_SNAPDIRS));
155 * Allocate a new inode with the passed id and ops.
157 static struct inode *
158 zfsctl_inode_alloc(zfs_sb_t *zsb, uint64_t id,
159 const struct file_operations *fops, const struct inode_operations *ops)
161 struct timespec now = current_fs_time(zsb->z_sb);
165 ip = new_inode(zsb->z_sb);
170 ASSERT3P(zp->z_dirlocks, ==, NULL);
171 ASSERT3P(zp->z_acl_cached, ==, NULL);
172 ASSERT3P(zp->z_xattr_cached, ==, NULL);
175 zp->z_atime_dirty = 0;
176 zp->z_zn_prefetch = 0;
192 zp->z_is_zvol = B_FALSE;
193 zp->z_is_mapped = B_FALSE;
194 zp->z_is_ctldir = B_TRUE;
195 zp->z_is_sa = B_FALSE;
197 ip->i_mode = (S_IFDIR | S_IRUGO | S_IXUGO);
200 ip->i_blkbits = SPA_MINBLOCKSHIFT;
207 if (insert_inode_locked(ip)) {
208 unlock_new_inode(ip);
213 mutex_enter(&zsb->z_znodes_lock);
214 list_insert_tail(&zsb->z_all_znodes, zp);
217 mutex_exit(&zsb->z_znodes_lock);
219 unlock_new_inode(ip);
225 * Lookup the inode with given id, it will be allocated if needed.
227 static struct inode *
228 zfsctl_inode_lookup(zfs_sb_t *zsb, unsigned long id,
229 const struct file_operations *fops, const struct inode_operations *ops)
231 struct inode *ip = NULL;
234 ip = ilookup(zsb->z_sb, id);
238 /* May fail due to concurrent zfsctl_inode_alloc() */
239 ip = zfsctl_inode_alloc(zsb, id, fops, ops);
246 * Free zfsctl inode specific structures, currently there are none.
249 zfsctl_inode_destroy(struct inode *ip)
255 * An inode is being evicted from the cache.
258 zfsctl_inode_inactive(struct inode *ip)
260 if (zfsctl_is_snapdir(ip))
261 zfsctl_snapdir_inactive(ip);
265 * Create the '.zfs' directory. This directory is cached as part of the VFS
266 * structure. This results in a hold on the zfs_sb_t. The code in zfs_umount()
267 * therefore checks against a vfs_count of 2 instead of 1. This reference
268 * is removed when the ctldir is destroyed in the unmount. All other entities
269 * under the '.zfs' directory are created dynamically as needed.
272 zfsctl_create(zfs_sb_t *zsb)
274 ASSERT(zsb->z_ctldir == NULL);
276 zsb->z_ctldir = zfsctl_inode_alloc(zsb, ZFSCTL_INO_ROOT,
277 &zpl_fops_root, &zpl_ops_root);
278 if (zsb->z_ctldir == NULL)
285 * Destroy the '.zfs' directory. Only called when the filesystem is unmounted.
288 zfsctl_destroy(zfs_sb_t *zsb)
291 zsb->z_ctldir = NULL;
295 * Given a root znode, retrieve the associated .zfs directory.
296 * Add a hold to the vnode and return it.
299 zfsctl_root(znode_t *zp)
301 ASSERT(zfs_has_ctldir(zp));
302 igrab(ZTOZSB(zp)->z_ctldir);
303 return (ZTOZSB(zp)->z_ctldir);
308 zfsctl_fid(struct inode *ip, fid_t *fidp)
310 znode_t *zp = ITOZ(ip);
311 zfs_sb_t *zsb = ITOZSB(ip);
312 uint64_t object = zp->z_id;
318 if (fidp->fid_len < SHORT_FID_LEN) {
319 fidp->fid_len = SHORT_FID_LEN;
324 zfid = (zfid_short_t *)fidp;
326 zfid->zf_len = SHORT_FID_LEN;
328 for (i = 0; i < sizeof (zfid->zf_object); i++)
329 zfid->zf_object[i] = (uint8_t)(object >> (8 * i));
331 /* .zfs znodes always have a generation number of 0 */
332 for (i = 0; i < sizeof (zfid->zf_gen); i++)
340 zfsctl_snapshot_zname(struct inode *ip, const char *name, int len, char *zname)
342 objset_t *os = ITOZSB(ip)->z_os;
344 if (snapshot_namecheck(name, NULL, NULL) != 0)
347 dmu_objset_name(os, zname);
348 if ((strlen(zname) + 1 + strlen(name)) >= len)
349 return (ENAMETOOLONG);
351 (void) strcat(zname, "@");
352 (void) strcat(zname, name);
358 zfsctl_snapshot_zpath(struct path *path, int len, char *zpath)
360 char *path_buffer, *path_ptr;
361 int path_len, error = 0;
363 path_buffer = kmem_alloc(len, KM_SLEEP);
365 path_ptr = d_path(path, path_buffer, len);
366 if (IS_ERR(path_ptr)) {
367 error = -PTR_ERR(path_ptr);
371 path_len = path_buffer + len - 1 - path_ptr;
372 if (path_len > len) {
377 memcpy(zpath, path_ptr, path_len);
378 zpath[path_len] = '\0';
380 kmem_free(path_buffer, len);
386 * Special case the handling of "..".
390 zfsctl_root_lookup(struct inode *dip, char *name, struct inode **ipp,
391 int flags, cred_t *cr, int *direntflags, pathname_t *realpnp)
393 zfs_sb_t *zsb = ITOZSB(dip);
398 if (strcmp(name, "..") == 0) {
399 *ipp = dip->i_sb->s_root->d_inode;
400 } else if (strcmp(name, ZFS_SNAPDIR_NAME) == 0) {
401 *ipp = zfsctl_inode_lookup(zsb, ZFSCTL_INO_SNAPDIR,
402 &zpl_fops_snapdir, &zpl_ops_snapdir);
403 } else if (strcmp(name, ZFS_SHAREDIR_NAME) == 0) {
404 *ipp = zfsctl_inode_lookup(zsb, ZFSCTL_INO_SHARES,
405 &zpl_fops_shares, &zpl_ops_shares);
419 * Lookup entry point for the 'snapshot' directory. Try to open the
420 * snapshot if it exist, creating the pseudo filesystem inode as necessary.
421 * Perform a mount of the associated dataset on top of the inode.
425 zfsctl_snapdir_lookup(struct inode *dip, char *name, struct inode **ipp,
426 int flags, cred_t *cr, int *direntflags, pathname_t *realpnp)
428 zfs_sb_t *zsb = ITOZSB(dip);
434 error = dmu_snapshot_id(zsb->z_os, name, &id);
440 *ipp = zfsctl_inode_lookup(zsb, ZFSCTL_INO_SNAPDIRS - id,
441 &simple_dir_operations, &simple_dir_inode_operations);
443 #ifdef HAVE_AUTOMOUNT
444 (*ipp)->i_flags |= S_AUTOMOUNT;
445 #endif /* HAVE_AUTOMOUNT */
456 zfsctl_rename_snap(zfs_sb_t *zsb, zfs_snapentry_t *sep, const char *name)
460 ASSERT(MUTEX_HELD(&zsb->z_ctldir_lock));
464 * Change the name in the AVL tree.
466 avl_remove(&zsb->z_ctldir_snaps, sep);
467 (void) strcpy(sep->se_name, name);
468 VERIFY(avl_find(&zsb->z_ctldir_snaps, sep, &where) == NULL);
469 avl_insert(&zsb->z_ctldir_snaps, sep, where);
473 * Renaming a directory under '.zfs/snapshot' will automatically trigger
474 * a rename of the snapshot to the new given name. The rename is confined
475 * to the '.zfs/snapshot' directory snapshots cannot be moved elsewhere.
479 zfsctl_snapdir_rename(struct inode *sdip, char *sname,
480 struct inode *tdip, char *tname, cred_t *cr, int flags)
482 zfs_sb_t *zsb = ITOZSB(sdip);
483 zfs_snapentry_t search, *sep;
485 char *to, *from, *real;
490 to = kmem_alloc(MAXNAMELEN, KM_SLEEP);
491 from = kmem_alloc(MAXNAMELEN, KM_SLEEP);
492 real = kmem_alloc(MAXNAMELEN, KM_SLEEP);
494 if (zsb->z_case == ZFS_CASE_INSENSITIVE) {
495 error = dmu_snapshot_realname(zsb->z_os, sname, real,
499 } else if (error != ENOTSUP) {
504 error = zfsctl_snapshot_zname(sdip, sname, MAXNAMELEN, from);
506 error = zfsctl_snapshot_zname(tdip, tname, MAXNAMELEN, to);
508 error = zfs_secpolicy_rename_perms(from, to, cr);
513 * Cannot move snapshots out of the snapdir.
521 * No-op when names are identical.
523 if (strcmp(sname, tname) == 0) {
528 mutex_enter(&zsb->z_ctldir_lock);
530 error = dmu_objset_rename(from, to, B_FALSE);
534 search.se_name = (char *)sname;
535 sep = avl_find(&zsb->z_ctldir_snaps, &search, &where);
537 zfsctl_rename_snap(zsb, sep, tname);
540 mutex_exit(&zsb->z_ctldir_lock);
542 kmem_free(from, MAXNAMELEN);
543 kmem_free(to, MAXNAMELEN);
544 kmem_free(real, MAXNAMELEN);
552 * Removing a directory under '.zfs/snapshot' will automatically trigger
553 * the removal of the snapshot with the given name.
557 zfsctl_snapdir_remove(struct inode *dip, char *name, cred_t *cr, int flags)
559 zfs_sb_t *zsb = ITOZSB(dip);
560 char *snapname, *real;
565 snapname = kmem_alloc(MAXNAMELEN, KM_SLEEP);
566 real = kmem_alloc(MAXNAMELEN, KM_SLEEP);
568 if (zsb->z_case == ZFS_CASE_INSENSITIVE) {
569 error = dmu_snapshot_realname(zsb->z_os, name, real,
573 } else if (error != ENOTSUP) {
578 error = zfsctl_snapshot_zname(dip, name, MAXNAMELEN, snapname);
580 error = zfs_secpolicy_destroy_perms(snapname, cr);
584 error = zfsctl_unmount_snapshot(zsb, name, MNT_FORCE);
585 if ((error == 0) || (error == ENOENT))
586 error = dmu_objset_destroy(snapname, B_FALSE);
588 kmem_free(snapname, MAXNAMELEN);
589 kmem_free(real, MAXNAMELEN);
597 * Creating a directory under '.zfs/snapshot' will automatically trigger
598 * the creation of a new snapshot with the given name.
602 zfsctl_snapdir_mkdir(struct inode *dip, char *dirname, vattr_t *vap,
603 struct inode **ipp, cred_t *cr, int flags)
605 zfs_sb_t *zsb = ITOZSB(dip);
609 dsname = kmem_alloc(MAXNAMELEN, KM_SLEEP);
611 if (snapshot_namecheck(dirname, NULL, NULL) != 0) {
616 dmu_objset_name(zsb->z_os, dsname);
618 error = zfs_secpolicy_snapshot_perms(dsname, cr);
623 error = dmu_objset_snapshot(dsname, dirname,
624 NULL, NULL, B_FALSE, B_FALSE, -1);
628 error = zfsctl_snapdir_lookup(dip, dirname, ipp,
632 kmem_free(dsname, MAXNAMELEN);
638 * When a .zfs/snapshot/<snapshot> inode is evicted they must be removed
639 * from the snapshot list. This will normally happen as part of the auto
640 * unmount, however in the case of a manual snapshot unmount this will be
641 * the only notification we receive.
644 zfsctl_snapdir_inactive(struct inode *ip)
646 zfs_sb_t *zsb = ITOZSB(ip);
647 zfs_snapentry_t *sep, *next;
649 mutex_enter(&zsb->z_ctldir_lock);
651 sep = avl_first(&zsb->z_ctldir_snaps);
652 while (sep != NULL) {
653 next = AVL_NEXT(&zsb->z_ctldir_snaps, sep);
655 if (sep->se_inode == ip) {
656 avl_remove(&zsb->z_ctldir_snaps, sep);
657 cancel_delayed_work_sync(&sep->se_work);
658 zfsctl_sep_free(sep);
664 mutex_exit(&zsb->z_ctldir_lock);
668 * Attempt to unmount a snapshot by making a call to user space.
669 * There is no assurance that this can or will succeed, is just a
670 * best effort. In the case where it does fail, perhaps because
671 * it's in use, the unmount will fail harmlessly.
673 #define SET_UNMOUNT_CMD \
674 "exec 0</dev/null " \
677 "umount -t zfs -n %s%s"
680 __zfsctl_unmount_snapshot(zfs_snapentry_t *sep, int flags)
682 char *argv[] = { "/bin/sh", "-c", NULL, NULL };
683 char *envp[] = { NULL };
686 argv[2] = kmem_asprintf(SET_UNMOUNT_CMD,
687 flags & MNT_FORCE ? "-f " : "", sep->se_path);
688 error = call_usermodehelper(argv[0], argv, envp, 1);
692 * The umount system utility will return 256 on error. We must
693 * assume this error is because the file system is busy so it is
694 * converted to the more sensible EBUSY.
700 * This was the result of a manual unmount, cancel the delayed work
701 * to prevent zfsctl_expire_snapshot() from attempting a unmount.
703 if ((error == 0) && !(flags & MNT_EXPIRE))
704 cancel_delayed_work(&sep->se_work);
710 zfsctl_unmount_snapshot(zfs_sb_t *zsb, char *name, int flags)
712 zfs_snapentry_t search;
713 zfs_snapentry_t *sep;
716 mutex_enter(&zsb->z_ctldir_lock);
718 search.se_name = name;
719 sep = avl_find(&zsb->z_ctldir_snaps, &search, NULL);
721 avl_remove(&zsb->z_ctldir_snaps, sep);
722 error = __zfsctl_unmount_snapshot(sep, flags);
724 avl_add(&zsb->z_ctldir_snaps, sep);
726 zfsctl_sep_free(sep);
731 mutex_exit(&zsb->z_ctldir_lock);
732 ASSERT3S(error, >=, 0);
738 * Traverse all mounted snapshots and attempt to unmount them. This
739 * is best effort, on failure EEXIST is returned and count will be set
740 * to the number of file snapshots which could not be unmounted.
743 zfsctl_unmount_snapshots(zfs_sb_t *zsb, int flags, int *count)
745 zfs_snapentry_t *sep, *next;
750 ASSERT(zsb->z_ctldir != NULL);
751 mutex_enter(&zsb->z_ctldir_lock);
753 sep = avl_first(&zsb->z_ctldir_snaps);
754 while (sep != NULL) {
755 next = AVL_NEXT(&zsb->z_ctldir_snaps, sep);
756 avl_remove(&zsb->z_ctldir_snaps, sep);
757 error = __zfsctl_unmount_snapshot(sep, flags);
758 if (error == EBUSY) {
759 avl_add(&zsb->z_ctldir_snaps, sep);
762 zfsctl_sep_free(sep);
768 mutex_exit(&zsb->z_ctldir_lock);
770 return ((*count > 0) ? EEXIST : 0);
773 #define SET_MOUNT_CMD \
774 "exec 0</dev/null " \
777 "mount -t zfs -n %s %s"
780 zfsctl_mount_snapshot(struct path *path, int flags)
782 struct dentry *dentry = path->dentry;
783 struct inode *ip = dentry->d_inode;
784 zfs_sb_t *zsb = ITOZSB(ip);
785 char *full_name, *full_path;
786 zfs_snapentry_t *sep;
787 zfs_snapentry_t search;
788 char *argv[] = { "/bin/sh", "-c", NULL, NULL };
789 char *envp[] = { NULL };
794 full_name = kmem_zalloc(MAXNAMELEN, KM_SLEEP);
795 full_path = kmem_zalloc(PATH_MAX, KM_SLEEP);
797 error = zfsctl_snapshot_zname(ip, dname(dentry), MAXNAMELEN, full_name);
801 error = zfsctl_snapshot_zpath(path, PATH_MAX, full_path);
806 * Attempt to mount the snapshot from user space. Normally this
807 * would be done using the vfs_kern_mount() function, however that
808 * function is marked GPL-only and cannot be used. On error we
809 * careful to log the real error to the console and return EISDIR
810 * to safely abort the automount. This should be very rare.
812 argv[2] = kmem_asprintf(SET_MOUNT_CMD, full_name, full_path);
813 error = call_usermodehelper(argv[0], argv, envp, 1);
816 printk("ZFS: Unable to automount %s at %s: %d\n",
817 full_name, full_path, error);
822 mutex_enter(&zsb->z_ctldir_lock);
825 * Ensure a previous entry does not exist, if it does safely remove
826 * it any cancel the outstanding expiration. This can occur when a
827 * snapshot is manually unmounted and then an automount is triggered.
829 search.se_name = full_name;
830 sep = avl_find(&zsb->z_ctldir_snaps, &search, NULL);
832 avl_remove(&zsb->z_ctldir_snaps, sep);
833 cancel_delayed_work_sync(&sep->se_work);
834 zfsctl_sep_free(sep);
837 sep = zfsctl_sep_alloc();
838 sep->se_name = full_name;
839 sep->se_path = full_path;
841 avl_add(&zsb->z_ctldir_snaps, sep);
843 spl_init_delayed_work(&sep->se_work, zfsctl_expire_snapshot, sep);
844 schedule_delayed_work(&sep->se_work, zfs_expire_snapshot * HZ);
846 mutex_exit(&zsb->z_ctldir_lock);
849 kmem_free(full_name, MAXNAMELEN);
850 kmem_free(full_path, PATH_MAX);
859 * Check if this super block has a matching objset id.
862 zfsctl_test_super(struct super_block *sb, void *objsetidp)
864 zfs_sb_t *zsb = sb->s_fs_info;
865 uint64_t objsetid = *(uint64_t *)objsetidp;
867 return (dmu_objset_id(zsb->z_os) == objsetid);
871 * Prevent a new super block from being allocated if an existing one
872 * could not be located. We only want to preform a lookup operation.
875 zfsctl_set_super(struct super_block *sb, void *objsetidp)
881 zfsctl_lookup_objset(struct super_block *sb, uint64_t objsetid, zfs_sb_t **zsbp)
883 zfs_sb_t *zsb = sb->s_fs_info;
884 struct super_block *sbp;
885 zfs_snapentry_t *sep;
889 ASSERT(zsb->z_ctldir != NULL);
891 mutex_enter(&zsb->z_ctldir_lock);
894 * Verify that the snapshot is mounted.
896 sep = avl_first(&zsb->z_ctldir_snaps);
897 while (sep != NULL) {
898 error = dmu_snapshot_id(zsb->z_os, sep->se_name, &id);
905 sep = AVL_NEXT(&zsb->z_ctldir_snaps, sep);
910 * Lookup the mounted root rather than the covered mount
911 * point. This may fail if the snapshot has just been
912 * unmounted by an unrelated user space process. This
913 * race cannot occur to an expired mount point because
914 * we hold the zsb->z_ctldir_lock to prevent the race.
916 sbp = sget(&zpl_fs_type, zfsctl_test_super,
917 zfsctl_set_super, &id);
919 error = -PTR_ERR(sbp);
921 *zsbp = sbp->s_fs_info;
922 deactivate_super(sbp);
928 mutex_exit(&zsb->z_ctldir_lock);
929 ASSERT3S(error, >=, 0);
936 zfsctl_shares_lookup(struct inode *dip, char *name, struct inode **ipp,
937 int flags, cred_t *cr, int *direntflags, pathname_t *realpnp)
939 zfs_sb_t *zsb = ITOZSB(dip);
946 if (zsb->z_shares_dir == 0) {
951 error = zfs_zget(zsb, zsb->z_shares_dir, &dzp);
957 error = zfs_lookup(ZTOI(dzp), name, &ip, 0, cr, NULL, NULL);
967 * Initialize the various pieces we'll need to create and manipulate .zfs
968 * directories. Currently this is unused but available.
976 * Cleanup the various pieces we needed for .zfs directories. In particular
977 * ensure the expiry timer is canceled safely.
984 module_param(zfs_expire_snapshot, int, 0644);
985 MODULE_PARM_DESC(zfs_expire_snapshot, "Seconds to expire .zfs/snapshot");