Improve too large physical ashift handling

When iterating through children physical ashifts for vdev, prefer
ones above the maximum logical ashift, that we can actually use,
but within the administrator defined maximum.

When selecting top-level vdev ashift, do not set it to the defined
maximum in case physical ashift is even higher, but just ignore one.
Using the maximum does not prevent misaligned writes, but reduces
space efficiency.  Since ZFS tries to write data sequentially and
aggregates the writes, in many cases large misanigned writes may be
not as bad as the space penalty otherwise.

Allow internal physical ashifts for vdevs higher than SHIFT_MAX.
May be one day allocator or aggregation could benefit from that.

Reduce zfs_vdev_max_auto_ashift default from 16 (64KB) to 14 (16KB),
so that ZFS may still use bigger ashifts up to SHIFT_MAX (64KB),
but only if it really has to or explicitly told to, but not as an
"optimization".

There are some read-intensive NVMe SSDs that report Preferred Write
Alignment of 64KB, and attempt to build RAIDZ2 of those leads to a
space inefficiency that can't be justified.  Instead these changes
make ZFS fall back to logical ashift of 12 (4KB) by default and
only warn user that it may be suboptimal for performance.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes #13798

module/zfs/vdev.c

@@ -136,7 +136,15 @@ int zfs_vdev_standard_sm_blksz = (1 << 17);
  */
 int zfs_nocacheflush = 0;
 
-uint64_t zfs_vdev_max_auto_ashift = ASHIFT_MAX;
+/*
+ * Maximum and minimum ashift values that can be automatically set based on
+ * vdev's physical ashift (disk's physical sector size).  While ASHIFT_MAX
+ * is higher than the maximum value, it is intentionally limited here to not
+ * excessively impact pool space efficiency.  Higher ashift values may still
+ * be forced by vdev logical ashift or by user via ashift property, but won't
+ * be set automatically as a performance optimization.
+ */
+uint64_t zfs_vdev_max_auto_ashift = 14;
 uint64_t zfs_vdev_min_auto_ashift = ASHIFT_MIN;
 
 void
@@ -1845,6 +1853,24 @@ vdev_set_deflate_ratio(vdev_t *vd)
 	}
 }
 
+/*
+ * Choose the best of two ashifts, preferring one between logical ashift
+ * (absolute minimum) and administrator defined maximum, otherwise take
+ * the biggest of the two.
+ */
+uint64_t
+vdev_best_ashift(uint64_t logical, uint64_t a, uint64_t b)
+{
+	if (a > logical && a <= zfs_vdev_max_auto_ashift) {
+		if (b <= logical || b > zfs_vdev_max_auto_ashift)
+			return (a);
+		else
+			return (MAX(a, b));
+	} else if (b <= logical || b > zfs_vdev_max_auto_ashift)
+		return (MAX(a, b));
+	return (b);
+}
+
 /*
  * Maximize performance by inflating the configured ashift for top level
  * vdevs to be as close to the physical ashift as possible while maintaining
@@ -1856,7 +1882,8 @@ vdev_ashift_optimize(vdev_t *vd)
 {
 	ASSERT(vd == vd->vdev_top);
 
-	if (vd->vdev_ashift < vd->vdev_physical_ashift) {
+	if (vd->vdev_ashift < vd->vdev_physical_ashift &&
+	    vd->vdev_physical_ashift <= zfs_vdev_max_auto_ashift) {
 		vd->vdev_ashift = MIN(
 		    MAX(zfs_vdev_max_auto_ashift, vd->vdev_ashift),
 		    MAX(zfs_vdev_min_auto_ashift,
@@ -4463,7 +4490,10 @@ vdev_get_stats_ex(vdev_t *vd, vdev_stat_t *vs, vdev_stat_ex_t *vsx)
 		vs->vs_configured_ashift = vd->vdev_top != NULL
 		    ? vd->vdev_top->vdev_ashift : vd->vdev_ashift;
 		vs->vs_logical_ashift = vd->vdev_logical_ashift;
-		vs->vs_physical_ashift = vd->vdev_physical_ashift;
+		if (vd->vdev_physical_ashift <= ASHIFT_MAX)
+			vs->vs_physical_ashift = vd->vdev_physical_ashift;
+		else
+			vs->vs_physical_ashift = 0;
 
 		/*
 		 * Report fragmentation and rebuild progress for top-level,

module/zfs/vdev_draid.c

@@ -1496,8 +1496,14 @@ vdev_draid_calculate_asize(vdev_t *vd, uint64_t *asizep, uint64_t *max_asizep,
 		asize = MIN(asize - 1, cvd->vdev_asize - 1) + 1;
 		max_asize = MIN(max_asize - 1, cvd->vdev_max_asize - 1) + 1;
 		logical_ashift = MAX(logical_ashift, cvd->vdev_ashift);
-		physical_ashift = MAX(physical_ashift,
-		    cvd->vdev_physical_ashift);
+	}
+	for (int c = 0; c < vd->vdev_children; c++) {
+		vdev_t *cvd = vd->vdev_child[c];
+
+		if (cvd->vdev_ops == &vdev_draid_spare_ops)
+			continue;
+		physical_ashift = vdev_best_ashift(logical_ashift,
+		    physical_ashift, cvd->vdev_physical_ashift);
 	}
 
 	*asizep = asize;

module/zfs/vdev_mirror.c

@@ -409,8 +409,14 @@ vdev_mirror_open(vdev_t *vd, uint64_t *asize, uint64_t *max_asize,
 		*asize = MIN(*asize - 1, cvd->vdev_asize - 1) + 1;
 		*max_asize = MIN(*max_asize - 1, cvd->vdev_max_asize - 1) + 1;
 		*logical_ashift = MAX(*logical_ashift, cvd->vdev_ashift);
-		*physical_ashift = MAX(*physical_ashift,
-		    cvd->vdev_physical_ashift);
+	}
+	for (int c = 0; c < vd->vdev_children; c++) {
+		vdev_t *cvd = vd->vdev_child[c];
+
+		if (cvd->vdev_open_error)
+			continue;
+		*physical_ashift = vdev_best_ashift(*logical_ashift,
+		    *physical_ashift, cvd->vdev_physical_ashift);
 	}
 
 	if (numerrors == vd->vdev_children) {

module/zfs/vdev_raidz.c

@@ -1527,8 +1527,14 @@ vdev_raidz_open(vdev_t *vd, uint64_t *asize, uint64_t *max_asize,
 		*asize = MIN(*asize - 1, cvd->vdev_asize - 1) + 1;
 		*max_asize = MIN(*max_asize - 1, cvd->vdev_max_asize - 1) + 1;
 		*logical_ashift = MAX(*logical_ashift, cvd->vdev_ashift);
-		*physical_ashift = MAX(*physical_ashift,
-		    cvd->vdev_physical_ashift);
+	}
+	for (c = 0; c < vd->vdev_children; c++) {
+		vdev_t *cvd = vd->vdev_child[c];
+
+		if (cvd->vdev_open_error != 0)
+			continue;
+		*physical_ashift = vdev_best_ashift(*logical_ashift,
+		    *physical_ashift, cvd->vdev_physical_ashift);
 	}
 
 	*asize *= vd->vdev_children;