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4334 Improve ZFS N-way mirror read performance

@@ -23,10 +23,11 @@
  * Use is subject to license terms.
  */
 
 /*
  * Copyright (c) 2013 by Delphix. All rights reserved.
+ * Copyright (c) 2013 Steven Hartland. All rights reserved.
  */
 
 #include <sys/zfs_context.h>
 #include <sys/spa.h>
 #include <sys/vdev_impl.h>

@@ -39,87 +40,175 @@
 
 typedef struct mirror_child {
         vdev_t          *mc_vd;
         uint64_t        mc_offset;
         int             mc_error;
+        int             mc_load;
         uint8_t         mc_tried;
         uint8_t         mc_skipped;
         uint8_t         mc_speculative;
 } mirror_child_t;
 
 typedef struct mirror_map {
+        int             *mm_preferred;
+        int             mm_preferred_cnt;
         int             mm_children;
-        int             mm_replacing;
-        int             mm_preferred;
-        int             mm_root;
-        mirror_child_t  mm_child[1];
+        boolean_t       mm_replacing;
+        boolean_t       mm_root;
+        mirror_child_t  mm_child[];
 } mirror_map_t;
 
-int vdev_mirror_shift = 21;
+int zfs_vdev_mirror_shift = 21;
+
+/*
+ * The load configuration settings below are tuned by default for
+ * the case where all devices are of the same rotational type.
+ *
+ * If there is a mixture of rotating and non-rotating media, setting
+ * non_rotating_seek_inc to 0 may well provide better results as it
+ * will direct more reads to the non-rotating vdevs which are more
+ * likely to have a higher performance.
+ */
+
+/* Rotating media load calculation configuration. */
+/* Rotating media load increment for non-seeking I/O's. */
+int zfs_vdev_mirror_rotating_inc = 0;
+
+/* Rotating media load increment for seeking I/O's. */
+int zfs_vdev_mirror_rotating_seek_inc = 5;
+
+/*
+ * Offset in bytes from the last I/O which triggers a reduced rotating media
+ * seek increment.
+ */
+int zfs_vdev_mirror_rotating_seek_offset = 1 * 1024 * 1024;
+
+/* Non-rotating media load calculation configuration. */
+/* Non-rotating media load increment for non-seeking I/O's. */
+int zfs_vdev_mirror_non_rotating_inc = 0;
+
+/* Non-rotating media load increment for seeking I/O's. */
+int zfs_vdev_mirror_non_rotating_seek_inc = 1;
+
+static inline size_t
+vdev_mirror_map_size(int children)
+{
+        return (offsetof(mirror_map_t, mm_child[children]) +
+            sizeof (int) * children);
+}
+
+static inline mirror_map_t *
+vdev_mirror_map_alloc(int children, boolean_t replacing, boolean_t root)
+{
+        mirror_map_t *mm;
+
+        mm = kmem_zalloc(vdev_mirror_map_size(children), KM_SLEEP);
+        mm->mm_children = children;
+        mm->mm_replacing = replacing;
+        mm->mm_root = root;
+        mm->mm_preferred = (int *)((uintptr_t)mm +
+            offsetof(mirror_map_t, mm_child[children]));
+
+        return (mm);
+}
 
 static void
 vdev_mirror_map_free(zio_t *zio)
 {
         mirror_map_t *mm = zio->io_vsd;
 
-        kmem_free(mm, offsetof(mirror_map_t, mm_child[mm->mm_children]));
+        kmem_free(mm, vdev_mirror_map_size(mm->mm_children));
 }
 
 static const zio_vsd_ops_t vdev_mirror_vsd_ops = {
         vdev_mirror_map_free,
         zio_vsd_default_cksum_report
 };
 
+/*
+ * Calculate and return the load of the specified vdev adjusted for a zio at
+ * the given offset.
+ *
+ * The calcuation takes into account the vdev's:
+ * 1. Rotation rate
+ * 2. The distance of zio_offset from the last queued request
+ */
+static int
+vdev_mirror_load(mirror_map_t *mm, vdev_t *vd, uint64_t zio_offset)
+{
+        uint64_t lastoffset;
+        int load;
+
+        /* All DVAs have equal weight at the root. */
+        if (mm->mm_root)
+                return (INT_MAX);
+
+        /*
+         * We don't return INT_MAX if the device is resilvering i.e.
+         * vdev_resilver_txg != 0 as when tested performance was slightly
+         * worse overall when resilvering with compared to without.
+         */
+
+        /* Standard load based on pending queue length. */
+        load = vdev_queue_length(vd);
+        lastoffset = vdev_queue_last_queued_offset(vd);
+
+        if (vd->vdev_rotation_rate == VDEV_RATE_NON_ROTATING) {
+                /* Non-rotating media. */
+                if (lastoffset == zio_offset)
+                        return (load + zfs_vdev_mirror_non_rotating_inc);
+
+                /*
+                 * Apply a seek penalty even for non-rotating devices as
+                 * sequential I/O'a can be aggregated into fewer operations
+                 * on the device, thus avoiding unnecessary per-command
+                 * overhead and boosting performance.
+                 */
+                return (load + zfs_vdev_mirror_non_rotating_seek_inc);
+        }
+
+        /* Rotating media I/O's which directly follow the last I/O. */
+        if (lastoffset == zio_offset)
+                return (load + zfs_vdev_mirror_rotating_inc);
+
+        /*
+         * Apply half the seek increment to I/O's within seek offset
+         * of the last I/O queued to this vdev as they should incure less
+         * of a seek increment.
+         */
+        if (ABS(lastoffset - zio_offset) <
+            zfs_vdev_mirror_rotating_seek_offset)
+                return (load + (zfs_vdev_mirror_rotating_seek_inc / 2));
+
+        /* Apply the full seek increment to all other I/O's. */
+        return (load + zfs_vdev_mirror_rotating_seek_inc);
+}
+
+
 static mirror_map_t *
-vdev_mirror_map_alloc(zio_t *zio)
+vdev_mirror_map_init(zio_t *zio)
 {
         mirror_map_t *mm = NULL;
         mirror_child_t *mc;
         vdev_t *vd = zio->io_vd;
-        int c, d;
+        int c;
 
         if (vd == NULL) {
                 dva_t *dva = zio->io_bp->blk_dva;
                 spa_t *spa = zio->io_spa;
 
-                c = BP_GET_NDVAS(zio->io_bp);
-
-                mm = kmem_zalloc(offsetof(mirror_map_t, mm_child[c]), KM_SLEEP);
-                mm->mm_children = c;
-                mm->mm_replacing = B_FALSE;
-                mm->mm_preferred = spa_get_random(c);
-                mm->mm_root = B_TRUE;
-
-                /*
-                 * Check the other, lower-index DVAs to see if they're on
-                 * the same vdev as the child we picked.  If they are, use
-                 * them since they are likely to have been allocated from
-                 * the primary metaslab in use at the time, and hence are
-                 * more likely to have locality with single-copy data.
-                 */
-                for (c = mm->mm_preferred, d = c - 1; d >= 0; d--) {
-                        if (DVA_GET_VDEV(&dva[d]) == DVA_GET_VDEV(&dva[c]))
-                                mm->mm_preferred = d;
-                }
-
+                mm = vdev_mirror_map_alloc(BP_GET_NDVAS(zio->io_bp), B_FALSE,
+                    B_TRUE);
                 for (c = 0; c < mm->mm_children; c++) {
                         mc = &mm->mm_child[c];
-
                         mc->mc_vd = vdev_lookup_top(spa, DVA_GET_VDEV(&dva[c]));
                         mc->mc_offset = DVA_GET_OFFSET(&dva[c]);
                 }
         } else {
-                c = vd->vdev_children;
-
-                mm = kmem_zalloc(offsetof(mirror_map_t, mm_child[c]), KM_SLEEP);
-                mm->mm_children = c;
-                mm->mm_replacing = (vd->vdev_ops == &vdev_replacing_ops ||
-                    vd->vdev_ops == &vdev_spare_ops);
-                mm->mm_preferred = mm->mm_replacing ? 0 :
-                    (zio->io_offset >> vdev_mirror_shift) % c;
-                mm->mm_root = B_FALSE;
-
+                mm = vdev_mirror_map_alloc(vd->vdev_children,
+                    (vd->vdev_ops == &vdev_replacing_ops ||
+                    vd->vdev_ops == &vdev_spare_ops), B_FALSE);
                 for (c = 0; c < mm->mm_children; c++) {
                         mc = &mm->mm_child[c];
                         mc->mc_vd = vd->vdev_child[c];
                         mc->mc_offset = zio->io_offset;
                 }

@@ -207,54 +296,125 @@
         mc->mc_tried = 1;
         mc->mc_skipped = 0;
 }
 
 /*
- * Try to find a child whose DTL doesn't contain the block we want to read.
+ * Check the other, lower-index DVAs to see if they're on the same
+ * vdev as the child we picked.  If they are, use them since they
+ * are likely to have been allocated from the primary metaslab in
+ * use at the time, and hence are more likely to have locality with
+ * single-copy data.
+ */
+static int
+vdev_mirror_dva_select(zio_t *zio, int preferred)
+{
+        dva_t *dva = zio->io_bp->blk_dva;
+        int c;
+
+        for (c = preferred - 1; c >= 0; c--) {
+                if (DVA_GET_VDEV(&dva[c]) == DVA_GET_VDEV(&dva[preferred]))
+                        preferred = c;
+        }
+        return (preferred);
+}
+
+static int
+vdev_mirror_preferred_child_randomize(zio_t *zio)
+{
+        mirror_map_t *mm = zio->io_vsd;
+        int p;
+
+        if (mm->mm_root) {
+                p = spa_get_random(mm->mm_preferred_cnt);
+                return (vdev_mirror_dva_select(zio, mm->mm_preferred[p]));
+        }
+
+        /*
+         * To ensure we don't always favour the first matching vdev,
+         * which could lead to wear leveling issues on SSD's, we
+         * use the I/O offset as a pseudo random seed into the vdevs
+         * which have the lowest load.
+         */
+        p = (zio->io_offset >> zfs_vdev_mirror_shift) % mm->mm_preferred_cnt;
+        return (mm->mm_preferred[p]);
+}
+
+/*
+ * Try to find a vdev whose DTL doesn't contain the block we want to read
+ * prefering vdevs based on determined load.
+ *
  * If we can't, try the read on any vdev we haven't already tried.
  */
 static int
 vdev_mirror_child_select(zio_t *zio)
 {
         mirror_map_t *mm = zio->io_vsd;
-        mirror_child_t *mc;
         uint64_t txg = zio->io_txg;
-        int i, c;
+        int c, lowest_load;
 
         ASSERT(zio->io_bp == NULL || BP_PHYSICAL_BIRTH(zio->io_bp) == txg);
 
-        /*
-         * Try to find a child whose DTL doesn't contain the block to read.
-         * If a child is known to be completely inaccessible (indicated by
-         * vdev_readable() returning B_FALSE), don't even try.
-         */
-        for (i = 0, c = mm->mm_preferred; i < mm->mm_children; i++, c++) {
-                if (c >= mm->mm_children)
-                        c = 0;
+        lowest_load = INT_MAX;
+        mm->mm_preferred_cnt = 0;
+        for (c = 0; c < mm->mm_children; c++) {
+                mirror_child_t *mc;
+
                 mc = &mm->mm_child[c];
                 if (mc->mc_tried || mc->mc_skipped)
                         continue;
+
                 if (!vdev_readable(mc->mc_vd)) {
                         mc->mc_error = SET_ERROR(ENXIO);
                         mc->mc_tried = 1;       /* don't even try */
                         mc->mc_skipped = 1;
                         continue;
                 }
-                if (!vdev_dtl_contains(mc->mc_vd, DTL_MISSING, txg, 1))
-                        return (c);
+
+                if (vdev_dtl_contains(mc->mc_vd, DTL_MISSING, txg, 1)) {
                 mc->mc_error = SET_ERROR(ESTALE);
                 mc->mc_skipped = 1;
                 mc->mc_speculative = 1;
+                        continue;
+                }
+
+                mc->mc_load = vdev_mirror_load(mm, mc->mc_vd, mc->mc_offset);
+                if (mc->mc_load > lowest_load)
+                        continue;
+
+                if (mc->mc_load < lowest_load) {
+                        lowest_load = mc->mc_load;
+                        mm->mm_preferred_cnt = 0;
+                }
+                mm->mm_preferred[mm->mm_preferred_cnt] = c;
+                mm->mm_preferred_cnt++;
+        }
+
+        if (mm->mm_preferred_cnt == 1) {
+                vdev_queue_register_last_queued_offset(
+                    mm->mm_child[mm->mm_preferred[0]].mc_vd, zio);
+                return (mm->mm_preferred[0]);
+        }
+
+        if (mm->mm_preferred_cnt > 1) {
+                int c = vdev_mirror_preferred_child_randomize(zio);
+
+                vdev_queue_register_last_queued_offset(mm->mm_child[c].mc_vd,
+                    zio);
+                return (c);
         }
 
         /*
          * Every device is either missing or has this txg in its DTL.
          * Look for any child we haven't already tried before giving up.
          */
-        for (c = 0; c < mm->mm_children; c++)
-                if (!mm->mm_child[c].mc_tried)
+        for (c = 0; c < mm->mm_children; c++) {
+                if (!mm->mm_child[c].mc_tried) {
+                        vdev_queue_register_last_queued_offset(
+                            mm->mm_child[c].mc_vd, zio);
                         return (c);
+                }
+        }
 
         /*
          * Every child failed.  There's no place left to look.
          */
         return (-1);

@@ -265,11 +425,11 @@
 {
         mirror_map_t *mm;
         mirror_child_t *mc;
         int c, children;
 
-        mm = vdev_mirror_map_alloc(zio);
+        mm = vdev_mirror_map_init(zio);
 
         if (zio->io_type == ZIO_TYPE_READ) {
                 if ((zio->io_flags & ZIO_FLAG_SCRUB) && !mm->mm_replacing) {
                         /*
                          * For scrubbing reads we need to allocate a read