diff --git a/mon/mon.js b/mon/mon.js
index 69b7c20a..a37d46e2 100644
--- a/mon/mon.js
+++ b/mon/mon.js
@@ -160,6 +160,8 @@ const etcd_tree = {
                 root_node?: 'rack1',
                 // restrict pool to OSDs having all of these tags
                 osd_tags?: 'nvme' | [ 'nvme', ... ],
+                // prefer to put primary on OSD with these tags
+                primary_affinity_tags?: 'nvme' | [ 'nvme', ... ],
             },
             ...
         }, */
@@ -903,27 +905,39 @@ class Mon
         return this.seed + 2147483648;
     }
 
-    pick_primary(pool_id, osd_set, up_osds)
+    pick_primary(pool_id, osd_set, up_osds, aff_osds)
     {
         let alive_set;
         if (this.state.config.pools[pool_id].scheme === 'replicated')
-            alive_set = osd_set.filter(osd_num => osd_num && up_osds[osd_num]);
+        {
+            // Prefer "affinity" OSDs
+            alive_set = osd_set.filter(osd_num => osd_num && aff_osds[osd_num]);
+            if (!alive_set.length)
+                alive_set = osd_set.filter(osd_num => osd_num && up_osds[osd_num]);
+        }
         else
         {
             // Prefer data OSDs for EC because they can actually read something without an additional network hop
             const pg_data_size = (this.state.config.pools[pool_id].pg_size||0) -
                 (this.state.config.pools[pool_id].parity_chunks||0);
-            alive_set = osd_set.slice(0, pg_data_size).filter(osd_num => osd_num && up_osds[osd_num]);
+            alive_set = osd_set.slice(0, pg_data_size).filter(osd_num => osd_num && aff_osds[osd_num]);
             if (!alive_set.length)
-                alive_set = osd_set.filter(osd_num => osd_num && up_osds[osd_num]);
+                alive_set = osd_set.filter(osd_num => osd_num && aff_osds[osd_num]);
+            if (!alive_set.length)
+            {
+                alive_set = osd_set.slice(0, pg_data_size).filter(osd_num => osd_num && up_osds[osd_num]);
+                if (!alive_set.length)
+                    alive_set = osd_set.filter(osd_num => osd_num && up_osds[osd_num]);
+            }
         }
         if (!alive_set.length)
             return 0;
         return alive_set[this.rng() % alive_set.length];
     }
 
-    save_new_pgs_txn(request, pool_id, up_osds, prev_pgs, new_pgs, pg_history)
+    save_new_pgs_txn(request, pool_id, up_osds, osd_tree, prev_pgs, new_pgs, pg_history)
     {
+        const aff_osds = this.get_affinity_osds(this.state.config.pools[pool_id], up_osds, osd_tree);
         const pg_items = {};
         this.reset_rng();
         new_pgs.map((osd_set, i) =>
@@ -931,7 +945,7 @@ class Mon
             osd_set = osd_set.map(osd_num => osd_num === LPOptimizer.NO_OSD ? 0 : osd_num);
             pg_items[i+1] = {
                 osd_set,
-                primary: this.pick_primary(pool_id, osd_set, up_osds),
+                primary: this.pick_primary(pool_id, osd_set, up_osds, aff_osds),
             };
             if (prev_pgs[i] && prev_pgs[i].join(' ') != osd_set.join(' ') &&
                 prev_pgs[i].filter(osd_num => osd_num).length > 0)
@@ -1062,6 +1076,13 @@ class Mon
                 console.log('Pool '+pool_id+' has invalid osd_tags (must be a string or array of strings)');
             return false;
         }
+        if (pool_cfg.primary_affinity_tags && typeof(pool_cfg.primary_affinity_tags) != 'string' &&
+            (!(pool_cfg.primary_affinity_tags instanceof Array) || pool_cfg.primary_affinity_tags.filter(t => typeof t != 'string').length > 0))
+        {
+            if (warn)
+                console.log('Pool '+pool_id+' has invalid primary_affinity_tags (must be a string or array of strings)');
+            return false;
+        }
         return true;
     }
 
@@ -1091,6 +1112,17 @@ class Mon
         }
     }
 
+    get_affinity_osds(pool_cfg, up_osds, osd_tree)
+    {
+        let aff_osds = up_osds;
+        if (pool_cfg.primary_affinity_tags)
+        {
+            aff_osds = { ...up_osds };
+            this.filter_osds_by_tags(osd_tree, { x: aff_osds }, pool_cfg.primary_affinity_tags);
+        }
+        return aff_osds;
+    }
+
     async recheck_pgs()
     {
         // Take configuration and state, check it against the stored configuration hash
@@ -1121,7 +1153,7 @@ class Mon
                     {
                         prev_pgs[pg-1] = this.state.config.pgs.items[pool_id][pg].osd_set;
                     }
-                    this.save_new_pgs_txn(etcd_request, pool_id, up_osds, prev_pgs, [], []);
+                    this.save_new_pgs_txn(etcd_request, pool_id, up_osds, osd_tree, prev_pgs, [], []);
                 }
             }
             for (const pool_id in this.state.config.pools)
@@ -1228,7 +1260,7 @@ class Mon
                     key: b64(this.etcd_prefix+'/pool/stats/'+pool_id),
                     value: b64(JSON.stringify(this.state.pool.stats[pool_id])),
                 } });
-                this.save_new_pgs_txn(etcd_request, pool_id, up_osds, real_prev_pgs, optimize_result.int_pgs, pg_history);
+                this.save_new_pgs_txn(etcd_request, pool_id, up_osds, osd_tree, real_prev_pgs, optimize_result.int_pgs, pg_history);
             }
             this.state.config.pgs.hash = tree_hash;
             await this.save_pg_config(etcd_request);
@@ -1245,13 +1277,14 @@ class Mon
                     continue;
                 }
                 const replicated = pool_cfg.scheme === 'replicated';
+                const aff_osds = this.get_affinity_osds(pool_cfg, up_osds, osd_tree);
                 this.reset_rng();
                 for (let pg_num = 1; pg_num <= pool_cfg.pg_count; pg_num++)
                 {
                     const pg_cfg = this.state.config.pgs.items[pool_id][pg_num];
                     if (pg_cfg)
                     {
-                        const new_primary = this.pick_primary(pool_id, pg_cfg.osd_set, up_osds);
+                        const new_primary = this.pick_primary(pool_id, pg_cfg.osd_set, up_osds, aff_osds);
                         if (pg_cfg.primary != new_primary)
                         {
                             console.log(