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diff --git a/src/common/shared_cache.hpp b/src/common/shared_cache.hpp
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@@ -0,0 +1,384 @@
+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*- 
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2004-2006 Sage Weil <sage@newdream.net>
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software 
+ * Foundation.  See file COPYING.
+ * 
+ */
+
+#ifndef CEPH_SHAREDCACHE_H
+#define CEPH_SHAREDCACHE_H
+
+#include <map>
+#include <list>
+#ifdef WITH_SEASTAR
+#include <boost/smart_ptr/local_shared_ptr.hpp>
+#else
+#include <memory>
+#endif
+#include "common/ceph_mutex.h"
+#include "common/ceph_context.h"
+#include "common/dout.h"
+#include "include/unordered_map.h"
+
+template <class K, class V>
+class SharedLRU {
+  CephContext *cct;
+#ifdef WITH_SEASTAR
+  using VPtr = boost::local_shared_ptr<V>;
+  using WeakVPtr = boost::weak_ptr<V>;
+#else
+  using VPtr = std::shared_ptr<V>;
+  using WeakVPtr = std::weak_ptr<V>;
+#endif
+  ceph::mutex lock;
+  size_t max_size;
+  ceph::condition_variable cond;
+  unsigned size;
+public:
+  int waiting;
+private:
+  using C = std::less<K>;
+  using H = std::hash<K>;
+  ceph::unordered_map<K, typename std::list<std::pair<K, VPtr> >::iterator, H> contents;
+  std::list<std::pair<K, VPtr> > lru;
+
+  std::map<K, std::pair<WeakVPtr, V*>, C> weak_refs;
+
+  void trim_cache(std::list<VPtr> *to_release) {
+    while (size > max_size) {
+      to_release->push_back(lru.back().second);
+      lru_remove(lru.back().first);
+    }
+  }
+
+  void lru_remove(const K& key) {
+    auto i = contents.find(key);
+    if (i == contents.end())
+      return;
+    lru.erase(i->second);
+    --size;
+    contents.erase(i);
+  }
+
+  void lru_add(const K& key, const VPtr& val, std::list<VPtr> *to_release) {
+    auto i = contents.find(key);
+    if (i != contents.end()) {
+      lru.splice(lru.begin(), lru, i->second);
+    } else {
+      ++size;
+      lru.push_front(make_pair(key, val));
+      contents[key] = lru.begin();
+      trim_cache(to_release);
+    }
+  }
+
+  void remove(const K& key, V *valptr) {
+    std::lock_guard l{lock};
+    auto i = weak_refs.find(key);
+    if (i != weak_refs.end() && i->second.second == valptr) {
+      weak_refs.erase(i);
+    }
+    cond.notify_all();
+  }
+
+  class Cleanup {
+  public:
+    SharedLRU<K, V> *cache;
+    K key;
+    Cleanup(SharedLRU<K, V> *cache, K key) : cache(cache), key(key) {}
+    void operator()(V *ptr) {
+      cache->remove(key, ptr);
+      delete ptr;
+    }
+  };
+
+public:
+  SharedLRU(CephContext *cct = NULL, size_t max_size = 20)
+    : cct(cct),
+      lock{ceph::make_mutex("SharedLRU::lock")},
+      max_size(max_size),
+      size(0), waiting(0) {
+    contents.rehash(max_size); 
+  }
+  
+  ~SharedLRU() {
+    contents.clear();
+    lru.clear();
+    if (!weak_refs.empty()) {
+      lderr(cct) << "leaked refs:\n";
+      dump_weak_refs(*_dout);
+      *_dout << dendl;
+      if (cct->_conf.get_val<bool>("debug_asserts_on_shutdown")) {
+	ceph_assert(weak_refs.empty());
+      }
+    }
+  }
+
+  int get_count() {
+    std::lock_guard locker{lock};
+    return size;
+  }
+
+  void set_cct(CephContext *c) {
+    cct = c;
+  }
+
+  void dump_weak_refs() {
+    lderr(cct) << "leaked refs:\n";
+    dump_weak_refs(*_dout);
+    *_dout << dendl;
+  }
+
+  void dump_weak_refs(std::ostream& out) {
+    for (const auto& [key, ref] : weak_refs) {
+      out << __func__ << " " << this << " weak_refs: "
+	  << key << " = " << ref.second
+	  << " with " << ref.first.use_count() << " refs"
+	  << std::endl;
+    }
+  }
+
+  //clear all strong reference from the lru.
+  void clear() {
+    while (true) {
+      VPtr val; // release any ref we have after we drop the lock
+      std::lock_guard locker{lock};
+      if (size == 0)
+        break;
+
+      val = lru.back().second;
+      lru_remove(lru.back().first);
+    }
+  }
+
+  void clear(const K& key) {
+    VPtr val; // release any ref we have after we drop the lock
+    {
+      std::lock_guard l{lock};
+      auto i = weak_refs.find(key);
+      if (i != weak_refs.end()) {
+	val = i->second.first.lock();
+      }
+      lru_remove(key);
+    }
+  }
+
+  /* Clears weakrefs in the interval [from, to] -- note that to is inclusive */
+  void clear_range(
+    const K& from,
+    const K& to) {
+    std::list<VPtr> vals; // release any refs we have after we drop the lock
+    {
+      std::lock_guard l{lock};
+      auto from_iter = weak_refs.lower_bound(from);
+      auto to_iter = weak_refs.upper_bound(to);
+      for (auto i = from_iter; i != to_iter; ) {
+	vals.push_back(i->second.first.lock());
+	lru_remove((i++)->first);
+      }
+    }
+  }
+
+
+  void purge(const K &key) {
+    VPtr val; // release any ref we have after we drop the lock
+    {
+      std::lock_guard l{lock};
+      auto i = weak_refs.find(key);
+      if (i != weak_refs.end()) {
+	val = i->second.first.lock();
+        weak_refs.erase(i);
+      }
+      lru_remove(key);
+    }
+  }
+
+  void set_size(size_t new_size) {
+    std::list<VPtr> to_release;
+    {
+      std::lock_guard l{lock};
+      max_size = new_size;
+      trim_cache(&to_release);
+    }
+  }
+
+  // Returns K key s.t. key <= k for all currently cached k,v
+  K cached_key_lower_bound() {
+    std::lock_guard l{lock};
+    return weak_refs.begin()->first;
+  }
+
+  VPtr lower_bound(const K& key) {
+    VPtr val;
+    std::list<VPtr> to_release;
+    {
+      std::unique_lock l{lock};
+      ++waiting;
+      cond.wait(l, [this, &key, &val, &to_release] {
+        if (weak_refs.empty()) {
+          return true;
+        }
+        auto i = weak_refs.lower_bound(key);
+        if (i == weak_refs.end()) {
+          --i;
+        }
+        if (val = i->second.first.lock(); val) {
+          lru_add(i->first, val, &to_release);
+          return true;
+        } else {
+          return false;
+        }
+      });
+      --waiting;
+    }
+    return val;
+  }
+  bool get_next(const K &key, std::pair<K, VPtr> *next) {
+    std::pair<K, VPtr> r;
+    {
+      std::lock_guard l{lock};
+      VPtr next_val;
+      typename std::map<K, std::pair<WeakVPtr, V*>, C>::iterator i = weak_refs.upper_bound(key);
+
+      while (i != weak_refs.end() &&
+	     !(next_val = i->second.first.lock()))
+	++i;
+
+      if (i == weak_refs.end())
+	return false;
+
+      if (next)
+	r = make_pair(i->first, next_val);
+    }
+    if (next)
+      *next = r;
+    return true;
+  }
+  bool get_next(const K &key, std::pair<K, V> *next) {
+    std::pair<K, VPtr> r;
+    bool found = get_next(key, &r);
+    if (!found || !next)
+      return found;
+    next->first = r.first;
+    ceph_assert(r.second);
+    next->second = *(r.second);
+    return found;
+  }
+
+  VPtr lookup(const K& key) {
+    VPtr val;
+    std::list<VPtr> to_release;
+    {
+      std::unique_lock l{lock};
+      ++waiting;
+      cond.wait(l, [this, &key, &val, &to_release] {
+        if (auto i = weak_refs.find(key); i != weak_refs.end()) {
+          if (val = i->second.first.lock(); val) {
+            lru_add(key, val, &to_release);
+            return true;
+          } else {
+            return false;
+          }
+        } else {
+          return true;
+        }
+      });
+      --waiting;
+    }
+    return val;
+  }
+  VPtr lookup_or_create(const K &key) {
+    VPtr val;
+    std::list<VPtr> to_release;
+    {
+      std::unique_lock l{lock};
+      cond.wait(l, [this, &key, &val] {
+        if (auto i = weak_refs.find(key); i != weak_refs.end()) {
+          if (val = i->second.first.lock(); val) {
+            return true;
+          } else {
+            return false;
+          }
+        } else {
+          return true;
+        }
+      });
+      if (!val) {
+        val = VPtr{new V{}, Cleanup{this, key}};
+        weak_refs.insert(make_pair(key, make_pair(val, val.get())));
+      }
+      lru_add(key, val, &to_release);
+    }
+    return val;
+  }
+
+  /**
+   * empty()
+   *
+   * Returns true iff there are no live references left to anything that has been
+   * in the cache.
+   */
+  bool empty() {
+    std::lock_guard l{lock};
+    return weak_refs.empty();
+  }
+
+  /***
+   * Inserts a key if not present, or bumps it to the front of the LRU if
+   * it is, and then gives you a reference to the value. If the key already
+   * existed, you are responsible for deleting the new value you tried to
+   * insert.
+   *
+   * @param key The key to insert
+   * @param value The value that goes with the key
+   * @param existed Set to true if the value was already in the
+   * map, false otherwise
+   * @return A reference to the map's value for the given key
+   */
+  VPtr add(const K& key, V *value, bool *existed = NULL) {
+    VPtr val;
+    std::list<VPtr> to_release;
+    {
+      typename std::map<K, std::pair<WeakVPtr, V*>, C>::iterator actual;
+      std::unique_lock l{lock};
+      cond.wait(l, [this, &key, &actual, &val] {
+	  actual = weak_refs.lower_bound(key);
+	  if (actual != weak_refs.end() && actual->first == key) {
+	    val = actual->second.first.lock();
+	    if (val) {
+	      return true;
+	    } else {
+	      return false;
+	    }
+	  } else {
+	    return true;
+	  }
+      });
+
+      if (val) {
+	if (existed) {
+	  *existed = true;
+	}
+      } else {
+	if (existed) {
+	  *existed = false;
+	}
+	val = VPtr(value, Cleanup(this, key));
+	weak_refs.insert(actual, make_pair(key, make_pair(val, value)));
+      }
+      lru_add(key, val, &to_release);
+    }
+    return val;
+  }
+
+  friend class SharedLRUTest;
+};
+
+#endif