diff --git a/scrub/phase8.c b/scrub/phase8.c
index 75400c9..e35bf11 100644
--- a/scrub/phase8.c
+++ b/scrub/phase8.c
@@ -45,27 +45,90 @@
 	return true;
 }
 
+/*
+ * Limit the amount of fstrim scanning that we let the kernel do in a single
+ * call so that we can implement decent progress reporting and CPU resource
+ * control.  Pick a prime number of gigabytes for interest.
+ */
+#define FSTRIM_MAX_BYTES	(11ULL << 30)
+
+/* Trim a certain range of the filesystem. */
+static int
+fstrim_fsblocks(
+	struct scrub_ctx	*ctx,
+	uint64_t		start_fsb,
+	uint64_t		fsbcount)
+{
+	uint64_t		start = cvt_off_fsb_to_b(&ctx->mnt, start_fsb);
+	uint64_t		len = cvt_off_fsb_to_b(&ctx->mnt, fsbcount);
+	int			error;
+
+	while (len > 0) {
+		uint64_t	run;
+
+		run = min(len, FSTRIM_MAX_BYTES);
+
+		error = fstrim(ctx, start, run);
+		if (error == EOPNOTSUPP) {
+			/* Pretend we finished all the work. */
+			progress_add(len);
+			return 0;
+		}
+		if (error) {
+			char		descr[DESCR_BUFSZ];
+
+			snprintf(descr, sizeof(descr) - 1,
+					_("fstrim start 0x%llx run 0x%llx"),
+					(unsigned long long)start,
+					(unsigned long long)run);
+			str_liberror(ctx, error, descr);
+			return error;
+		}
+
+		progress_add(run);
+		len -= run;
+		start += run;
+	}
+
+	return 0;
+}
+
+/* Trim each AG on the data device. */
+static int
+fstrim_datadev(
+	struct scrub_ctx	*ctx)
+{
+	struct xfs_fsop_geom	*geo = &ctx->mnt.fsgeom;
+	uint64_t		fsbno;
+	int			error;
+
+	for (fsbno = 0; fsbno < geo->datablocks; fsbno += geo->agblocks) {
+		uint64_t	fsbcount;
+
+		/*
+		 * Make sure that trim calls do not cross AG boundaries so that
+		 * the kernel only performs one log force (and takes one AGF
+		 * lock) per call.
+		 */
+		progress_add(geo->blocksize);
+		fsbcount = min(geo->datablocks - fsbno, geo->agblocks);
+		error = fstrim_fsblocks(ctx, fsbno, fsbcount);
+		if (error)
+			return error;
+	}
+
+	return 0;
+}
+
 /* Trim the filesystem, if desired. */
 int
 phase8_func(
 	struct scrub_ctx	*ctx)
 {
-	int			error;
-
 	if (!fstrim_ok(ctx))
 		return 0;
 
-	error = fstrim(ctx);
-	if (error == EOPNOTSUPP)
-		return 0;
-
-	if (error) {
-		str_liberror(ctx, error, _("fstrim"));
-		return error;
-	}
-
-	progress_add(1);
-	return 0;
+	return fstrim_datadev(ctx);
 }
 
 /* Estimate how much work we're going to do. */
@@ -76,12 +139,13 @@
 	unsigned int		*nr_threads,
 	int			*rshift)
 {
-	*items = 0;
-
-	if (fstrim_ok(ctx))
-		*items = 1;
-
+	if (fstrim_ok(ctx)) {
+		*items = cvt_off_fsb_to_b(&ctx->mnt,
+				ctx->mnt.fsgeom.datablocks);
+	} else {
+		*items = 0;
+	}
 	*nr_threads = 1;
-	*rshift = 0;
+	*rshift = 30; /* GiB */
 	return 0;
 }
diff --git a/scrub/vfs.c b/scrub/vfs.c
index bcfd4f4..cc958ba 100644
--- a/scrub/vfs.c
+++ b/scrub/vfs.c
@@ -298,11 +298,15 @@
 /* Call FITRIM to trim all the unused space in a filesystem. */
 int
 fstrim(
-	struct scrub_ctx	*ctx)
+	struct scrub_ctx	*ctx,
+	uint64_t		start,
+	uint64_t		len)
 {
-	struct fstrim_range	range = {0};
+	struct fstrim_range	range = {
+		.start		= start,
+		.len		= len,
+	};
 
-	range.len = ULLONG_MAX;
 	if (ioctl(ctx->mnt.fd, FITRIM, &range) == 0)
 		return 0;
 	if (errno == EOPNOTSUPP || errno == ENOTTY)
diff --git a/scrub/vfs.h b/scrub/vfs.h
index a8a4d72..1af8d80 100644
--- a/scrub/vfs.h
+++ b/scrub/vfs.h
@@ -24,6 +24,6 @@
 int scan_fs_tree(struct scrub_ctx *ctx, scan_fs_tree_dir_fn dir_fn,
 		scan_fs_tree_dirent_fn dirent_fn, void *arg);
 
-int fstrim(struct scrub_ctx *ctx);
+int fstrim(struct scrub_ctx *ctx, uint64_t start, uint64_t len);
 
 #endif /* XFS_SCRUB_VFS_H_ */