Illumos #3122 zfs destroy filesystem should prefetch blocks
[zfs.git] / module / zfs / dmu_traverse.c
index 89cbfad..370a28a 100644 (file)
@@ -19,8 +19,8 @@
  * CDDL HEADER END
  */
 /*
- * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
- * Use is subject to license terms.
+ * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012 by Delphix. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
 #include <sys/spa.h>
 #include <sys/zio.h>
 #include <sys/dmu_impl.h>
+#include <sys/sa.h>
+#include <sys/sa_impl.h>
 #include <sys/callb.h>
 
-#define        SET_BOOKMARK(zb, objset, object, level, blkid)  \
-{                                                       \
-       (zb)->zb_objset = objset;                       \
-       (zb)->zb_object = object;                       \
-       (zb)->zb_level = level;                         \
-       (zb)->zb_blkid = blkid;                         \
-}
+int zfs_pd_blks_max = 100;
 
-struct prefetch_data {
+typedef struct prefetch_data {
        kmutex_t pd_mtx;
        kcondvar_t pd_cv;
        int pd_blks_max;
@@ -51,47 +47,49 @@ struct prefetch_data {
        int pd_flags;
        boolean_t pd_cancel;
        boolean_t pd_exited;
-};
+} prefetch_data_t;
 
-struct traverse_data {
+typedef struct traverse_data {
        spa_t *td_spa;
        uint64_t td_objset;
        blkptr_t *td_rootbp;
        uint64_t td_min_txg;
+       zbookmark_t *td_resume;
        int td_flags;
-       struct prefetch_data *td_pfd;
+       prefetch_data_t *td_pfd;
        blkptr_cb_t *td_func;
        void *td_arg;
-};
+} traverse_data_t;
 
-static int traverse_dnode(struct traverse_data *td, const dnode_phys_t *dnp,
+static int traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
+    arc_buf_t *buf, uint64_t objset, uint64_t object);
+static void prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *,
     arc_buf_t *buf, uint64_t objset, uint64_t object);
 
-/* ARGSUSED */
-static void
+static int
 traverse_zil_block(zilog_t *zilog, blkptr_t *bp, void *arg, uint64_t claim_txg)
 {
-       struct traverse_data *td = arg;
+       traverse_data_t *td = arg;
        zbookmark_t zb;
 
        if (bp->blk_birth == 0)
-               return;
+               return (0);
 
        if (claim_txg == 0 && bp->blk_birth >= spa_first_txg(td->td_spa))
-               return;
+               return (0);
+
+       SET_BOOKMARK(&zb, td->td_objset, ZB_ZIL_OBJECT, ZB_ZIL_LEVEL,
+           bp->blk_cksum.zc_word[ZIL_ZC_SEQ]);
 
-       zb.zb_objset = td->td_objset;
-       zb.zb_object = 0;
-       zb.zb_level = -1;
-       zb.zb_blkid = bp->blk_cksum.zc_word[ZIL_ZC_SEQ];
-       VERIFY(0 == td->td_func(td->td_spa, bp, &zb, NULL, td->td_arg));
+       (void) td->td_func(td->td_spa, zilog, bp, NULL, &zb, NULL, td->td_arg);
+
+       return (0);
 }
 
-/* ARGSUSED */
-static void
+static int
 traverse_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
 {
-       struct traverse_data *td = arg;
+       traverse_data_t *td = arg;
 
        if (lrc->lrc_txtype == TX_WRITE) {
                lr_write_t *lr = (lr_write_t *)lrc;
@@ -99,28 +97,29 @@ traverse_zil_record(zilog_t *zilog, lr_t *lrc, void *arg, uint64_t claim_txg)
                zbookmark_t zb;
 
                if (bp->blk_birth == 0)
-                       return;
+                       return (0);
 
                if (claim_txg == 0 || bp->blk_birth < claim_txg)
-                       return;
+                       return (0);
 
-               zb.zb_objset = td->td_objset;
-               zb.zb_object = lr->lr_foid;
-               zb.zb_level = BP_GET_LEVEL(bp);
-               zb.zb_blkid = lr->lr_offset / BP_GET_LSIZE(bp);
-               VERIFY(0 == td->td_func(td->td_spa, bp, &zb, NULL, td->td_arg));
+               SET_BOOKMARK(&zb, td->td_objset, lr->lr_foid,
+                   ZB_ZIL_LEVEL, lr->lr_offset / BP_GET_LSIZE(bp));
+
+               (void) td->td_func(td->td_spa, zilog, bp, NULL, &zb, NULL,
+                   td->td_arg);
        }
+       return (0);
 }
 
 static void
-traverse_zil(struct traverse_data *td, zil_header_t *zh)
+traverse_zil(traverse_data_t *td, zil_header_t *zh)
 {
        uint64_t claim_txg = zh->zh_claim_txg;
        zilog_t *zilog;
 
        /*
         * We only want to visit blocks that have been claimed but not yet
-        * replayed (or, in read-only mode, blocks that *would* be claimed).
+        * replayed; plus, in read-only mode, blocks that are already stable.
         */
        if (claim_txg == 0 && spa_writeable(td->td_spa))
                return;
@@ -133,17 +132,104 @@ traverse_zil(struct traverse_data *td, zil_header_t *zh)
        zil_free(zilog);
 }
 
+typedef enum resume_skip {
+       RESUME_SKIP_ALL,
+       RESUME_SKIP_NONE,
+       RESUME_SKIP_CHILDREN
+} resume_skip_t;
+
+/*
+ * Returns RESUME_SKIP_ALL if td indicates that we are resuming a traversal and
+ * the block indicated by zb does not need to be visited at all. Returns
+ * RESUME_SKIP_CHILDREN if we are resuming a post traversal and we reach the
+ * resume point. This indicates that this block should be visited but not its
+ * children (since they must have been visited in a previous traversal).
+ * Otherwise returns RESUME_SKIP_NONE.
+ */
+static resume_skip_t
+resume_skip_check(traverse_data_t *td, const dnode_phys_t *dnp,
+    const zbookmark_t *zb)
+{
+       if (td->td_resume != NULL && !ZB_IS_ZERO(td->td_resume)) {
+               /*
+                * If we already visited this bp & everything below,
+                * don't bother doing it again.
+                */
+               if (zbookmark_is_before(dnp, zb, td->td_resume))
+                       return (RESUME_SKIP_ALL);
+
+               /*
+                * If we found the block we're trying to resume from, zero
+                * the bookmark out to indicate that we have resumed.
+                */
+               ASSERT3U(zb->zb_object, <=, td->td_resume->zb_object);
+               if (bcmp(zb, td->td_resume, sizeof (*zb)) == 0) {
+                       bzero(td->td_resume, sizeof (*zb));
+                       if (td->td_flags & TRAVERSE_POST)
+                               return (RESUME_SKIP_CHILDREN);
+               }
+       }
+       return (RESUME_SKIP_NONE);
+}
+
+static void
+traverse_pause(traverse_data_t *td, const zbookmark_t *zb)
+{
+       ASSERT(td->td_resume != NULL);
+       ASSERT0(zb->zb_level);
+       bcopy(zb, td->td_resume, sizeof (*td->td_resume));
+}
+
+static void
+traverse_prefetch_metadata(traverse_data_t *td,
+    arc_buf_t *pbuf, const blkptr_t *bp, const zbookmark_t *zb)
+{
+       uint32_t flags = ARC_NOWAIT | ARC_PREFETCH;
+
+       if (!(td->td_flags & TRAVERSE_PREFETCH_METADATA))
+               return;
+       /*
+        * If we are in the process of resuming, don't prefetch, because
+        * some children will not be needed (and in fact may have already
+        * been freed).
+        */
+       if (td->td_resume != NULL && !ZB_IS_ZERO(td->td_resume))
+               return;
+       if (BP_IS_HOLE(bp) || bp->blk_birth <= td->td_min_txg)
+               return;
+       if (BP_GET_LEVEL(bp) == 0 && BP_GET_TYPE(bp) != DMU_OT_DNODE)
+               return;
+
+       (void) arc_read(NULL, td->td_spa, bp,
+           pbuf, NULL, NULL, ZIO_PRIORITY_ASYNC_READ,
+           ZIO_FLAG_CANFAIL, &flags, zb);
+}
+
 static int
-traverse_visitbp(struct traverse_data *td, const dnode_phys_t *dnp,
-    arc_buf_t *pbuf, blkptr_t *bp, const zbookmark_t *zb)
+traverse_visitbp(traverse_data_t *td, const dnode_phys_t *dnp,
+    arc_buf_t *pbuf, const blkptr_t *bp, const zbookmark_t *zb)
 {
        zbookmark_t czb;
-       int err = 0;
+       int err = 0, lasterr = 0;
        arc_buf_t *buf = NULL;
-       struct prefetch_data *pd = td->td_pfd;
+       prefetch_data_t *pd = td->td_pfd;
+       boolean_t hard = td->td_flags & TRAVERSE_HARD;
+       boolean_t pause = B_FALSE;
 
-       if (bp->blk_birth == 0) {
-               err = td->td_func(td->td_spa, NULL, zb, dnp, td->td_arg);
+       switch (resume_skip_check(td, dnp, zb)) {
+       case RESUME_SKIP_ALL:
+               return (0);
+       case RESUME_SKIP_CHILDREN:
+               goto post;
+       case RESUME_SKIP_NONE:
+               break;
+       default:
+               ASSERT(0);
+       }
+
+       if (BP_IS_HOLE(bp)) {
+               err = td->td_func(td->td_spa, NULL, NULL, pbuf, zb, dnp,
+                   td->td_arg);
                return (err);
        }
 
@@ -163,9 +249,14 @@ traverse_visitbp(struct traverse_data *td, const dnode_phys_t *dnp,
        }
 
        if (td->td_flags & TRAVERSE_PRE) {
-               err = td->td_func(td->td_spa, bp, zb, dnp, td->td_arg);
-               if (err)
-                       return (err);
+               err = td->td_func(td->td_spa, NULL, bp, pbuf, zb, dnp,
+                   td->td_arg);
+               if (err == TRAVERSE_VISIT_NO_CHILDREN)
+                       return (0);
+               if (err == ERESTART)
+                       pause = B_TRUE; /* handle pausing at a common point */
+               if (err != 0)
+                       goto post;
        }
 
        if (BP_GET_LEVEL(bp) > 0) {
@@ -174,62 +265,96 @@ traverse_visitbp(struct traverse_data *td, const dnode_phys_t *dnp,
                blkptr_t *cbp;
                int epb = BP_GET_LSIZE(bp) >> SPA_BLKPTRSHIFT;
 
-               err = arc_read(NULL, td->td_spa, bp, pbuf,
+               err = dsl_read(NULL, td->td_spa, bp, pbuf,
                    arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
                if (err)
                        return (err);
+               cbp = buf->b_data;
+
+               for (i = 0; i < epb; i++) {
+                       SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
+                           zb->zb_level - 1,
+                           zb->zb_blkid * epb + i);
+                       traverse_prefetch_metadata(td, buf, &cbp[i], &czb);
+               }
 
                /* recursively visitbp() blocks below this */
-               cbp = buf->b_data;
-               for (i = 0; i < epb; i++, cbp++) {
+               for (i = 0; i < epb; i++) {
                        SET_BOOKMARK(&czb, zb->zb_objset, zb->zb_object,
                            zb->zb_level - 1,
                            zb->zb_blkid * epb + i);
-                       err = traverse_visitbp(td, dnp, buf, cbp, &czb);
-                       if (err)
-                               break;
+                       err = traverse_visitbp(td, dnp, buf, &cbp[i], &czb);
+                       if (err) {
+                               if (!hard)
+                                       break;
+                               lasterr = err;
+                       }
                }
        } else if (BP_GET_TYPE(bp) == DMU_OT_DNODE) {
                uint32_t flags = ARC_WAIT;
                int i;
                int epb = BP_GET_LSIZE(bp) >> DNODE_SHIFT;
 
-               err = arc_read(NULL, td->td_spa, bp, pbuf,
+               err = dsl_read(NULL, td->td_spa, bp, pbuf,
                    arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
                if (err)
                        return (err);
+               dnp = buf->b_data;
+
+               for (i = 0; i < epb; i++) {
+                       prefetch_dnode_metadata(td, &dnp[i], buf, zb->zb_objset,
+                           zb->zb_blkid * epb + i);
+               }
 
                /* recursively visitbp() blocks below this */
-               dnp = buf->b_data;
-               for (i = 0; i < epb && err == 0; i++, dnp++) {
-                       err = traverse_dnode(td, dnp, buf, zb->zb_objset,
+               for (i = 0; i < epb; i++) {
+                       err = traverse_dnode(td, &dnp[i], buf, zb->zb_objset,
                            zb->zb_blkid * epb + i);
-                       if (err)
-                               break;
+                       if (err) {
+                               if (!hard)
+                                       break;
+                               lasterr = err;
+                       }
                }
        } else if (BP_GET_TYPE(bp) == DMU_OT_OBJSET) {
                uint32_t flags = ARC_WAIT;
                objset_phys_t *osp;
                dnode_phys_t *dnp;
 
-               err = arc_read_nolock(NULL, td->td_spa, bp,
+               err = dsl_read_nolock(NULL, td->td_spa, bp,
                    arc_getbuf_func, &buf,
                    ZIO_PRIORITY_ASYNC_READ, ZIO_FLAG_CANFAIL, &flags, zb);
                if (err)
                        return (err);
 
                osp = buf->b_data;
-               traverse_zil(td, &osp->os_zil_header);
-
                dnp = &osp->os_meta_dnode;
-               err = traverse_dnode(td, dnp, buf, zb->zb_objset, 0);
+               prefetch_dnode_metadata(td, dnp, buf, zb->zb_objset,
+                   DMU_META_DNODE_OBJECT);
+               if (arc_buf_size(buf) >= sizeof (objset_phys_t)) {
+                       prefetch_dnode_metadata(td, &osp->os_userused_dnode,
+                           buf, zb->zb_objset, DMU_USERUSED_OBJECT);
+                       prefetch_dnode_metadata(td, &osp->os_groupused_dnode,
+                           buf, zb->zb_objset, DMU_USERUSED_OBJECT);
+               }
+
+               err = traverse_dnode(td, dnp, buf, zb->zb_objset,
+                   DMU_META_DNODE_OBJECT);
+               if (err && hard) {
+                       lasterr = err;
+                       err = 0;
+               }
                if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
                        dnp = &osp->os_userused_dnode;
                        err = traverse_dnode(td, dnp, buf, zb->zb_objset,
                            DMU_USERUSED_OBJECT);
                }
+               if (err && hard) {
+                       lasterr = err;
+                       err = 0;
+               }
                if (err == 0 && arc_buf_size(buf) >= sizeof (objset_phys_t)) {
                        dnp = &osp->os_groupused_dnode;
                        err = traverse_dnode(td, dnp, buf, zb->zb_objset,
@@ -240,35 +365,78 @@ traverse_visitbp(struct traverse_data *td, const dnode_phys_t *dnp,
        if (buf)
                (void) arc_buf_remove_ref(buf, &buf);
 
-       if (err == 0 && (td->td_flags & TRAVERSE_POST))
-               err = td->td_func(td->td_spa, bp, zb, dnp, td->td_arg);
+post:
+       if (err == 0 && lasterr == 0 && (td->td_flags & TRAVERSE_POST)) {
+               err = td->td_func(td->td_spa, NULL, bp, pbuf, zb, dnp,
+                   td->td_arg);
+               if (err == ERESTART)
+                       pause = B_TRUE;
+       }
 
-       return (err);
+       if (pause && td->td_resume != NULL) {
+               ASSERT3U(err, ==, ERESTART);
+               ASSERT(!hard);
+               traverse_pause(td, zb);
+       }
+
+       return (err != 0 ? err : lasterr);
+}
+
+static void
+prefetch_dnode_metadata(traverse_data_t *td, const dnode_phys_t *dnp,
+    arc_buf_t *buf, uint64_t objset, uint64_t object)
+{
+       int j;
+       zbookmark_t czb;
+
+       for (j = 0; j < dnp->dn_nblkptr; j++) {
+               SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
+               traverse_prefetch_metadata(td, buf, &dnp->dn_blkptr[j], &czb);
+       }
+
+       if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
+               SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
+               traverse_prefetch_metadata(td, buf, &dnp->dn_spill, &czb);
+       }
 }
 
 static int
-traverse_dnode(struct traverse_data *td, const dnode_phys_t *dnp,
+traverse_dnode(traverse_data_t *td, const dnode_phys_t *dnp,
     arc_buf_t *buf, uint64_t objset, uint64_t object)
 {
-       int j, err = 0;
+       int j, err = 0, lasterr = 0;
        zbookmark_t czb;
+       boolean_t hard = (td->td_flags & TRAVERSE_HARD);
 
        for (j = 0; j < dnp->dn_nblkptr; j++) {
                SET_BOOKMARK(&czb, objset, object, dnp->dn_nlevels - 1, j);
-               err = traverse_visitbp(td, dnp, buf,
-                   (blkptr_t *)&dnp->dn_blkptr[j], &czb);
-               if (err)
-                       break;
+               err = traverse_visitbp(td, dnp, buf, &dnp->dn_blkptr[j], &czb);
+               if (err) {
+                       if (!hard)
+                               break;
+                       lasterr = err;
+               }
        }
-       return (err);
+
+       if (dnp->dn_flags & DNODE_FLAG_SPILL_BLKPTR) {
+               SET_BOOKMARK(&czb, objset, object, 0, DMU_SPILL_BLKID);
+               err = traverse_visitbp(td, dnp, buf, &dnp->dn_spill, &czb);
+               if (err) {
+                       if (!hard)
+                               return (err);
+                       lasterr = err;
+               }
+       }
+       return (err != 0 ? err : lasterr);
 }
 
 /* ARGSUSED */
 static int
-traverse_prefetcher(spa_t *spa, blkptr_t *bp, const zbookmark_t *zb,
-    const dnode_phys_t *dnp, void *arg)
+traverse_prefetcher(spa_t *spa, zilog_t *zilog, const blkptr_t *bp,
+    arc_buf_t *pbuf, const zbookmark_t *zb, const dnode_phys_t *dnp,
+    void *arg)
 {
-       struct prefetch_data *pfd = arg;
+       prefetch_data_t *pfd = arg;
        uint32_t aflags = ARC_NOWAIT | ARC_PREFETCH;
 
        ASSERT(pfd->pd_blks_fetched >= 0);
@@ -276,7 +444,8 @@ traverse_prefetcher(spa_t *spa, blkptr_t *bp, const zbookmark_t *zb,
                return (EINTR);
 
        if (bp == NULL || !((pfd->pd_flags & TRAVERSE_PREFETCH_DATA) ||
-           BP_GET_TYPE(bp) == DMU_OT_DNODE || BP_GET_LEVEL(bp) > 0))
+           BP_GET_TYPE(bp) == DMU_OT_DNODE || BP_GET_LEVEL(bp) > 0) ||
+           BP_GET_TYPE(bp) == DMU_OT_INTENT_LOG)
                return (0);
 
        mutex_enter(&pfd->pd_mtx);
@@ -286,7 +455,7 @@ traverse_prefetcher(spa_t *spa, blkptr_t *bp, const zbookmark_t *zb,
        cv_broadcast(&pfd->pd_cv);
        mutex_exit(&pfd->pd_mtx);
 
-       (void) arc_read_nolock(NULL, spa, bp, NULL, NULL,
+       (void) dsl_read(NULL, spa, bp, pbuf, NULL, NULL,
            ZIO_PRIORITY_ASYNC_READ,
            ZIO_FLAG_CANFAIL | ZIO_FLAG_SPECULATIVE,
            &aflags, zb);
@@ -297,15 +466,16 @@ traverse_prefetcher(spa_t *spa, blkptr_t *bp, const zbookmark_t *zb,
 static void
 traverse_prefetch_thread(void *arg)
 {
-       struct traverse_data *td_main = arg;
-       struct traverse_data td = *td_main;
+       traverse_data_t *td_main = arg;
+       traverse_data_t td = *td_main;
        zbookmark_t czb;
 
        td.td_func = traverse_prefetcher;
        td.td_arg = td_main->td_pfd;
        td.td_pfd = NULL;
 
-       SET_BOOKMARK(&czb, td.td_objset, 0, -1, 0);
+       SET_BOOKMARK(&czb, td.td_objset,
+           ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
        (void) traverse_visitbp(&td, NULL, NULL, td.td_rootbp, &czb);
 
        mutex_enter(&td_main->td_pfd->pd_mtx);
@@ -319,45 +489,76 @@ traverse_prefetch_thread(void *arg)
  * in syncing context).
  */
 static int
-traverse_impl(spa_t *spa, uint64_t objset, blkptr_t *rootbp,
-    uint64_t txg_start, int flags, blkptr_cb_t func, void *arg)
+traverse_impl(spa_t *spa, dsl_dataset_t *ds, uint64_t objset, blkptr_t *rootbp,
+    uint64_t txg_start, zbookmark_t *resume, int flags,
+    blkptr_cb_t func, void *arg)
 {
-       struct traverse_data td;
-       struct prefetch_data pd = { 0 };
-       zbookmark_t czb;
+       traverse_data_t *td;
+       prefetch_data_t *pd;
+       zbookmark_t *czb;
        int err;
 
-       td.td_spa = spa;
-       td.td_objset = objset;
-       td.td_rootbp = rootbp;
-       td.td_min_txg = txg_start;
-       td.td_func = func;
-       td.td_arg = arg;
-       td.td_pfd = &pd;
-       td.td_flags = flags;
-
-       pd.pd_blks_max = 100;
-       pd.pd_flags = flags;
-       mutex_init(&pd.pd_mtx, NULL, MUTEX_DEFAULT, NULL);
-       cv_init(&pd.pd_cv, NULL, CV_DEFAULT, NULL);
-
-       if (!(flags & TRAVERSE_PREFETCH) ||
+       ASSERT(ds == NULL || objset == ds->ds_object);
+       ASSERT(!(flags & TRAVERSE_PRE) || !(flags & TRAVERSE_POST));
+
+       /*
+        * The data prefetching mechanism (the prefetch thread) is incompatible
+        * with resuming from a bookmark.
+        */
+       ASSERT(resume == NULL || !(flags & TRAVERSE_PREFETCH_DATA));
+
+       td = kmem_alloc(sizeof(traverse_data_t), KM_PUSHPAGE);
+       pd = kmem_zalloc(sizeof(prefetch_data_t), KM_PUSHPAGE);
+       czb = kmem_alloc(sizeof(zbookmark_t), KM_PUSHPAGE);
+
+       td->td_spa = spa;
+       td->td_objset = objset;
+       td->td_rootbp = rootbp;
+       td->td_min_txg = txg_start;
+       td->td_resume = resume;
+       td->td_func = func;
+       td->td_arg = arg;
+       td->td_pfd = pd;
+       td->td_flags = flags;
+
+       pd->pd_blks_max = zfs_pd_blks_max;
+       pd->pd_flags = flags;
+       mutex_init(&pd->pd_mtx, NULL, MUTEX_DEFAULT, NULL);
+       cv_init(&pd->pd_cv, NULL, CV_DEFAULT, NULL);
+
+       /* See comment on ZIL traversal in dsl_scan_visitds. */
+       if (ds != NULL && !dsl_dataset_is_snapshot(ds)) {
+               objset_t *os;
+
+               err = dmu_objset_from_ds(ds, &os);
+               if (err)
+                       return (err);
+
+               traverse_zil(td, &os->os_zil_header);
+       }
+
+       if (!(flags & TRAVERSE_PREFETCH_DATA) ||
            0 == taskq_dispatch(system_taskq, traverse_prefetch_thread,
-           &td, TQ_NOQUEUE))
-               pd.pd_exited = B_TRUE;
+           td, TQ_NOQUEUE))
+               pd->pd_exited = B_TRUE;
+
+       SET_BOOKMARK(czb, td->td_objset,
+           ZB_ROOT_OBJECT, ZB_ROOT_LEVEL, ZB_ROOT_BLKID);
+       err = traverse_visitbp(td, NULL, NULL, rootbp, czb);
 
-       SET_BOOKMARK(&czb, objset, 0, -1, 0);
-       err = traverse_visitbp(&td, NULL, NULL, rootbp, &czb);
+       mutex_enter(&pd->pd_mtx);
+       pd->pd_cancel = B_TRUE;
+       cv_broadcast(&pd->pd_cv);
+       while (!pd->pd_exited)
+               cv_wait(&pd->pd_cv, &pd->pd_mtx);
+       mutex_exit(&pd->pd_mtx);
 
-       mutex_enter(&pd.pd_mtx);
-       pd.pd_cancel = B_TRUE;
-       cv_broadcast(&pd.pd_cv);
-       while (!pd.pd_exited)
-               cv_wait(&pd.pd_cv, &pd.pd_mtx);
-       mutex_exit(&pd.pd_mtx);
+       mutex_destroy(&pd->pd_mtx);
+       cv_destroy(&pd->pd_cv);
 
-       mutex_destroy(&pd.pd_mtx);
-       cv_destroy(&pd.pd_cv);
+       kmem_free(czb, sizeof(zbookmark_t));
+       kmem_free(pd, sizeof(struct prefetch_data));
+       kmem_free(td, sizeof(struct traverse_data));
 
        return (err);
 }
@@ -370,51 +571,84 @@ int
 traverse_dataset(dsl_dataset_t *ds, uint64_t txg_start, int flags,
     blkptr_cb_t func, void *arg)
 {
-       return (traverse_impl(ds->ds_dir->dd_pool->dp_spa, ds->ds_object,
-           &ds->ds_phys->ds_bp, txg_start, flags, func, arg));
+       return (traverse_impl(ds->ds_dir->dd_pool->dp_spa, ds, ds->ds_object,
+           &ds->ds_phys->ds_bp, txg_start, NULL, flags, func, arg));
+}
+
+int
+traverse_dataset_destroyed(spa_t *spa, blkptr_t *blkptr,
+    uint64_t txg_start, zbookmark_t *resume, int flags,
+    blkptr_cb_t func, void *arg)
+{
+       return (traverse_impl(spa, NULL, ZB_DESTROYED_OBJSET,
+           blkptr, txg_start, resume, flags, func, arg));
 }
 
 /*
  * NB: pool must not be changing on-disk (eg, from zdb or sync context).
  */
 int
-traverse_pool(spa_t *spa, blkptr_cb_t func, void *arg)
+traverse_pool(spa_t *spa, uint64_t txg_start, int flags,
+    blkptr_cb_t func, void *arg)
 {
-       int err;
+       int err, lasterr = 0;
        uint64_t obj;
        dsl_pool_t *dp = spa_get_dsl(spa);
        objset_t *mos = dp->dp_meta_objset;
+       boolean_t hard = (flags & TRAVERSE_HARD);
 
        /* visit the MOS */
-       err = traverse_impl(spa, 0, spa_get_rootblkptr(spa),
-           0, TRAVERSE_PRE, func, arg);
+       err = traverse_impl(spa, NULL, 0, spa_get_rootblkptr(spa),
+           txg_start, NULL, flags, func, arg);
        if (err)
                return (err);
 
        /* visit each dataset */
-       for (obj = 1; err == 0; err = dmu_object_next(mos, &obj, FALSE, 0)) {
+       for (obj = 1; err == 0 || (err != ESRCH && hard);
+           err = dmu_object_next(mos, &obj, FALSE, txg_start)) {
                dmu_object_info_t doi;
 
                err = dmu_object_info(mos, obj, &doi);
-               if (err)
-                       return (err);
+               if (err) {
+                       if (!hard)
+                               return (err);
+                       lasterr = err;
+                       continue;
+               }
 
                if (doi.doi_type == DMU_OT_DSL_DATASET) {
                        dsl_dataset_t *ds;
+                       uint64_t txg = txg_start;
+
                        rw_enter(&dp->dp_config_rwlock, RW_READER);
                        err = dsl_dataset_hold_obj(dp, obj, FTAG, &ds);
                        rw_exit(&dp->dp_config_rwlock);
-                       if (err)
-                               return (err);
-                       err = traverse_dataset(ds,
-                           ds->ds_phys->ds_prev_snap_txg, TRAVERSE_PRE,
-                           func, arg);
+                       if (err) {
+                               if (!hard)
+                                       return (err);
+                               lasterr = err;
+                               continue;
+                       }
+                       if (ds->ds_phys->ds_prev_snap_txg > txg)
+                               txg = ds->ds_phys->ds_prev_snap_txg;
+                       err = traverse_dataset(ds, txg, flags, func, arg);
                        dsl_dataset_rele(ds, FTAG);
-                       if (err)
-                               return (err);
+                       if (err) {
+                               if (!hard)
+                                       return (err);
+                               lasterr = err;
+                       }
                }
        }
        if (err == ESRCH)
                err = 0;
-       return (err);
+       return (err != 0 ? err : lasterr);
 }
+
+#if defined(_KERNEL) && defined(HAVE_SPL)
+EXPORT_SYMBOL(traverse_dataset);
+EXPORT_SYMBOL(traverse_pool);
+
+module_param(zfs_pd_blks_max, int, 0644);
+MODULE_PARM_DESC(zfs_pd_blks_max, "Max number of blocks to prefetch");
+#endif