Do not report bytes skipped by scan as issued.

Scan process may skip blocks based on their birth time, DVA, etc.
Traditionally those blocks were accounted as issued, that caused
reporting of hugely over-inflated numbers, having nothing to do
with actual disk I/O.  This change utilizes never used field in
struct dsl_scan_phys to account such skipped bytes, allowing to
report how much data were actually scrubbed/resilvered and what
is the actual I/O speed.  While formally it is an on-disk format
change, it should be compatible both ways, so should not need a
feature flag.

This should partially address the same issue as c85ac731a0, but
from a different perspective, complementing it.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Akash B <akash-b@hpe.com>
Signed-off-by:  Alexander Motin <mav@FreeBSD.org>
Sponsored by:   iXsystems, Inc.
Closes #15007

module/zfs/dsl_scan.c

@@ -573,7 +573,8 @@ dsl_scan_init(dsl_pool_t *dp, uint64_t txg)
 		 * counter to how far we've scanned. We know we're consistent
 		 * up to here.
 		 */
-		scn->scn_issued_before_pass = scn->scn_phys.scn_examined;
+		scn->scn_issued_before_pass = scn->scn_phys.scn_examined -
+		    scn->scn_phys.scn_skipped;
 
 		if (dsl_scan_is_running(scn) &&
 		    spa_prev_software_version(dp->dp_spa) < SPA_VERSION_SCAN) {
@@ -4362,7 +4363,7 @@ dsl_scan_sync(dsl_pool_t *dp, dmu_tx_t *tx)
 	 * Disabled by default, set zfs_scan_report_txgs to report
 	 * average performance over the last zfs_scan_report_txgs TXGs.
 	 */
-	if (!dsl_scan_is_paused_scrub(scn) && zfs_scan_report_txgs != 0 &&
+	if (zfs_scan_report_txgs != 0 &&
 	    tx->tx_txg % zfs_scan_report_txgs == 0) {
 		scn->scn_issued_before_pass += spa->spa_scan_pass_issued;
 		spa_scan_stat_init(spa);
@@ -4564,6 +4565,15 @@ count_block_issued(spa_t *spa, const blkptr_t *bp, boolean_t all)
 	    all ? BP_GET_ASIZE(bp) : DVA_GET_ASIZE(&bp->blk_dva[0]));
 }
 
+static void
+count_block_skipped(dsl_scan_t *scn, const blkptr_t *bp, boolean_t all)
+{
+	if (BP_IS_EMBEDDED(bp))
+		return;
+	atomic_add_64(&scn->scn_phys.scn_skipped,
+	    all ? BP_GET_ASIZE(bp) : DVA_GET_ASIZE(&bp->blk_dva[0]));
+}
+
 static void
 count_block(zfs_all_blkstats_t *zab, const blkptr_t *bp)
 {
@@ -4709,7 +4719,7 @@ dsl_scan_scrub_cb(dsl_pool_t *dp,
 	count_block(dp->dp_blkstats, bp);
 	if (phys_birth <= scn->scn_phys.scn_min_txg ||
 	    phys_birth >= scn->scn_phys.scn_max_txg) {
-		count_block_issued(spa, bp, B_TRUE);
+		count_block_skipped(scn, bp, B_TRUE);
 		return (0);
 	}
 
@@ -4750,7 +4760,7 @@ dsl_scan_scrub_cb(dsl_pool_t *dp,
 	if (needs_io && !zfs_no_scrub_io) {
 		dsl_scan_enqueue(dp, bp, zio_flags, zb);
 	} else {
-		count_block_issued(spa, bp, B_TRUE);
+		count_block_skipped(scn, bp, B_TRUE);
 	}
 
 	/* do not relocate this block */
@@ -5119,9 +5129,9 @@ dsl_scan_freed_dva(spa_t *spa, const blkptr_t *bp, int dva_i)
 		ASSERT(range_tree_contains(queue->q_exts_by_addr, start, size));
 		range_tree_remove_fill(queue->q_exts_by_addr, start, size);
 
-		/* count the block as though we issued it */
+		/* count the block as though we skipped it */
 		sio2bp(sio, &tmpbp);
-		count_block_issued(spa, &tmpbp, B_FALSE);
+		count_block_skipped(scn, &tmpbp, B_FALSE);
 
 		sio_free(sio);
 	}

module/zfs/spa_misc.c

@@ -2611,7 +2611,7 @@ spa_scan_get_stats(spa_t *spa, pool_scan_stat_t *ps)
 	ps->pss_end_time = scn->scn_phys.scn_end_time;
 	ps->pss_to_examine = scn->scn_phys.scn_to_examine;
 	ps->pss_examined = scn->scn_phys.scn_examined;
-	ps->pss_to_process = scn->scn_phys.scn_to_process;
+	ps->pss_skipped = scn->scn_phys.scn_skipped;
 	ps->pss_processed = scn->scn_phys.scn_processed;
 	ps->pss_errors = scn->scn_phys.scn_errors;
 

module/zfs/vdev_rebuild.c

@@ -571,8 +571,10 @@ vdev_rebuild_range(vdev_rebuild_t *vr, uint64_t start, uint64_t size)
 	vdev_rebuild_blkptr_init(&blk, vd, start, size);
 	uint64_t psize = BP_GET_PSIZE(&blk);
 
-	if (!vdev_dtl_need_resilver(vd, &blk.blk_dva[0], psize, TXG_UNKNOWN))
+	if (!vdev_dtl_need_resilver(vd, &blk.blk_dva[0], psize, TXG_UNKNOWN)) {
+		vr->vr_pass_bytes_skipped += size;
 		return (0);
+	}
 
 	mutex_enter(&vr->vr_io_lock);
 
@@ -786,6 +788,7 @@ vdev_rebuild_thread(void *arg)
 	vr->vr_pass_start_time = gethrtime();
 	vr->vr_pass_bytes_scanned = 0;
 	vr->vr_pass_bytes_issued = 0;
+	vr->vr_pass_bytes_skipped = 0;
 
 	uint64_t update_est_time = gethrtime();
 	vdev_rebuild_update_bytes_est(vd, 0);
@@ -1153,6 +1156,7 @@ vdev_rebuild_get_stats(vdev_t *tvd, vdev_rebuild_stat_t *vrs)
 		    vr->vr_pass_start_time);
 		vrs->vrs_pass_bytes_scanned = vr->vr_pass_bytes_scanned;
 		vrs->vrs_pass_bytes_issued = vr->vr_pass_bytes_issued;
+		vrs->vrs_pass_bytes_skipped = vr->vr_pass_bytes_skipped;
 		mutex_exit(&tvd->vdev_rebuild_lock);
 	}