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 <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by:	Alexander Motin <[email protected]>
Sponsored by:	iXsystems, Inc.
Closes openzfs#13798

Author: amotin

include/sys/vdev_impl.h

@@ -641,6 +641,7 @@ extern int vdev_obsolete_counts_are_precise(vdev_t *vd, boolean_t *are_precise);
  */
 int vdev_checkpoint_sm_object(vdev_t *vd, uint64_t *sm_obj);
 void vdev_metaslab_group_create(vdev_t *vd);
+uint64_t vdev_best_ashift(uint64_t logical, uint64_t a, uint64_t b);
 
 /*
  * Vdev ashift optimization tunables

man/man4/zfs.4

@@ -347,9 +347,12 @@ When a vdev is added, target this number of metaslabs per top-level vdev.
 .It Sy zfs_vdev_default_ms_shift Ns = Ns Sy 29 Po 512 MiB Pc Pq int
 Default limit for metaslab size.
 .
-.It Sy zfs_vdev_max_auto_ashift Ns = Ns Sy ASHIFT_MAX Po 16 Pc Pq ulong
+.It Sy zfs_vdev_max_auto_ashift Ns = Ns Sy 14 Pq ulong
 Maximum ashift used when optimizing for logical \[->] physical sector size on new
 top-level vdevs.
+May be increased up to
+.Sy ASHIFT_MAX Po 16 Pc ,
+but this may negatively impact pool space efficiency.
 .
 .It Sy zfs_vdev_min_auto_ashift Ns = Ns Sy ASHIFT_MIN Po 9 Pc Pq ulong
 Minimum ashift used when creating new top-level vdevs.

module/os/freebsd/zfs/vdev_geom.c

@@ -955,8 +955,7 @@ vdev_geom_open(vdev_t *vd, uint64_t *psize, uint64_t *max_psize,
 	*logical_ashift = highbit(MAX(pp->sectorsize, SPA_MINBLOCKSIZE)) - 1;
 	*physical_ashift = 0;
 	if (pp->stripesize && pp->stripesize > (1 << *logical_ashift) &&
-	    ISP2(pp->stripesize) && pp->stripesize <= (1 << ASHIFT_MAX) &&
-	    pp->stripeoffset == 0)
+	    ISP2(pp->stripesize) && pp->stripeoffset == 0)
 		*physical_ashift = highbit(pp->stripesize) - 1;
 
 	/*

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;

tests/zfs-tests/include/tunables.cfg

@@ -81,7 +81,9 @@ TRIM_TXG_BATCH			trim.txg_batch			zfs_trim_txg_batch
 TXG_HISTORY			txg.history			zfs_txg_history
 TXG_TIMEOUT			txg.timeout			zfs_txg_timeout
 UNLINK_SUSPEND_PROGRESS		UNSUPPORTED			zfs_unlink_suspend_progress
+VDEV_FILE_LOGICAL_ASHIFT	vdev.file.logical_ashift	vdev_file_logical_ashift
 VDEV_FILE_PHYSICAL_ASHIFT	vdev.file.physical_ashift	vdev_file_physical_ashift
+VDEV_MAX_AUTO_ASHIFT		vdev.max_auto_ashift		zfs_vdev_max_auto_ashift
 VDEV_MIN_MS_COUNT		vdev.min_ms_count		zfs_vdev_min_ms_count
 VDEV_VALIDATE_SKIP		vdev.validate_skip		vdev_validate_skip
 VOL_INHIBIT_DEV			UNSUPPORTED			zvol_inhibit_dev

tests/zfs-tests/tests/functional/cli_root/zpool_add/add-o_ashift.ksh

@@ -57,7 +57,9 @@ disk2=$TEST_BASE_DIR/disk2
 log_must mkfile $SIZE $disk1
 log_must mkfile $SIZE $disk2
 
+logical_ashift=$(get_tunable VDEV_FILE_LOGICAL_ASHIFT)
 orig_ashift=$(get_tunable VDEV_FILE_PHYSICAL_ASHIFT)
+max_auto_ashift=$(get_tunable VDEV_MAX_AUTO_ASHIFT)
 
 typeset ashifts=("9" "10" "11" "12" "13" "14" "15" "16")
 for ashift in ${ashifts[@]}
@@ -77,7 +79,8 @@ do
 	log_must zpool create $TESTPOOL $disk1
 	log_must set_tunable64 VDEV_FILE_PHYSICAL_ASHIFT $ashift
 	log_must zpool add $TESTPOOL $disk2
-	log_must verify_ashift $disk2 $ashift
+	exp=$(( (ashift <= max_auto_ashift) ? ashift : logical_ashift ))
+	log_must verify_ashift $disk2 $exp
 
 	# clean things for the next run
 	log_must set_tunable64 VDEV_FILE_PHYSICAL_ASHIFT $orig_ashift