diff --git a/src/os/bluestore/BlueStore.cc b/src/os/bluestore/BlueStore.cc
index 627ee25fdc8dc..32f8071ba82ef 100644
--- a/src/os/bluestore/BlueStore.cc
+++ b/src/os/bluestore/BlueStore.cc
@@ -1776,11 +1776,18 @@ void BlueStore::ExtentMap::fault_range(
 {
   dout(30) << __func__ << " 0x" << std::hex << offset << "~" << length
 	   << std::dec << dendl;
-  auto p = seek_shard(offset);
+  auto start = seek_shard(offset);
   auto last = seek_shard(offset + length);
+
+  if (start < 0)
+    return;
+
+  assert(last >= start);
   bool first_key = true;
   string key;
-  while (p != shards.end()) {
+  while (start <= last) {
+    assert((size_t)start < shards.size());
+    auto p = &shards[start];
     if (!p->loaded) {
       if (first_key) {
         get_extent_shard_key(onode->key, p->offset, &key);
@@ -1801,10 +1808,7 @@ void BlueStore::ExtentMap::fault_range(
       assert(p->dirty == false);
       assert(v.length() == p->shard_info->bytes);
     }
-    if (p == last) {
-      break;
-    }
-    ++p;
+    ++start;
   }
 }
 
@@ -1820,12 +1824,18 @@ void BlueStore::ExtentMap::dirty_range(
     inline_bl.clear();
     return;
   }
-  auto p = seek_shard(offset);
+  auto start = seek_shard(offset);
   auto last = seek_shard(offset + length);
-  while (p != shards.end()) {
+  if (start < 0)
+    return;
+
+  assert(last >= start);
+  while (start <= last) {
+    assert((size_t)start < shards.size());
+    auto p = &shards[start];
     if (!p->loaded) {
       dout(20) << __func__ << " shard 0x" << std::hex << p->offset << std::dec
-	       << " is not loaded, can't mark dirty" << dendl;
+               << " is not loaded, can't mark dirty" << dendl;
       assert(0 == "can't mark unloaded shard dirty");
     }
     if (!p->dirty) {
@@ -1833,10 +1843,7 @@ void BlueStore::ExtentMap::dirty_range(
 	       << std::dec << " dirty" << dendl;
       p->dirty = true;
     }
-    if (p == last) {
-      break;
-    }
-    ++p;
+    ++start;
   }
 }
 
diff --git a/src/os/bluestore/BlueStore.h b/src/os/bluestore/BlueStore.h
index 362e0316cf9f1..b3da8012cbfd1 100644
--- a/src/os/bluestore/BlueStore.h
+++ b/src/os/bluestore/BlueStore.h
@@ -574,21 +574,28 @@ class BlueStore : public ObjectStore,
     /// initialize Shards from the onode
     void init_shards(Onode *on, bool loaded, bool dirty);
 
-    /// return shard containing offset
-    vector<Shard>::iterator seek_shard(uint32_t offset) {
-      // fixme: we could do a binary search here
-      // we want the right-most shard that has an offset <= @offset.
-      vector<Shard>::iterator p = shards.begin();
-      while (p != shards.end() &&
-	     p->offset <= offset) {
-	++p;
-      }
-      if (p != shards.begin()) {
-	assert(p == shards.end() || p->offset > offset);
-	--p;
-	assert(p->offset <= offset);
+    /// return index of shard containing offset
+    /// or -1 if not found
+    int seek_shard(uint32_t offset) {
+      size_t end = shards.size();
+      size_t mid, left = 0;
+      size_t right = end; // one passed the right end
+
+      while (left < right) {
+        mid = left + (right - left) / 2;
+        if (offset >= shards[mid].offset) {
+          size_t next = mid + 1;
+          if (next >= end || offset < shards[next].offset)
+            return mid;
+          //continue to search forwards
+          left = next;
+        } else {
+          //continue to search backwards
+          right = mid;
+        }
       }
-      return p;
+
+      return -1; // not found
     }
 
     /// ensure that a range of the map is loaded