title: How to debug the error "OOM command not allowed when used memory > 'maxmemory'" in Redis?
body: <p>I'm getting "OOM command not allowed" when trying to set a key, 
  <code>maxmemory</code> is set to 500M with <code>maxmemory-policy</code> "volatile-lru", I'm setting TTL for each key sent to redis.</p>
  
  <p><code>INFO</code> command returns : <code>used_memory_human:809.22M</code></p>
  
  <ol>
  <li>If maxmemory is set to 500M, how did I reached 809M ?</li>
  <li><code>INFO</code> command does not show any Keyspaces , how is it possible ? </li>
  <li><code>KEYS *</code> returns "(empty list or set)" ,I've tried to change db number , still no keys found.</li>
  </ol>
  
  <p>Here is info command output:</p>
  
  <pre><code>redis-cli -p 6380
  redis 127.0.0.1:6380&gt; info
  # Server
  redis_version:2.6.4
  redis_git_sha1:00000000
  redis_git_dirty:0
  redis_mode:standalone
  os:Linux 2.6.32-358.14.1.el6.x86_64 x86_64
  arch_bits:64
  multiplexing_api:epoll
  gcc_version:4.4.7
  process_id:28291
  run_id:229a2ee688bdbf677eaed24620102e7060725350
  tcp_port:6380
  uptime_in_seconds:1492488
  uptime_in_days:17
  lru_clock:1429357
  
  # Clients
  connected_clients:1
  client_longest_output_list:0
  client_biggest_input_buf:0
  blocked_clients:0
  
  # Memory
  used_memory:848529904
  used_memory_human:809.22M
  used_memory_rss:863551488
  used_memory_peak:848529192
  used_memory_peak_human:809.22M
  used_memory_lua:31744
  mem_fragmentation_ratio:1.02
  mem_allocator:jemalloc-3.0.0
  
  # Persistence
  loading:0
  rdb_changes_since_last_save:0
  rdb_bgsave_in_progress:0
  rdb_last_save_time:1375949883
  rdb_last_bgsave_status:ok
  rdb_last_bgsave_time_sec:-1
  rdb_current_bgsave_time_sec:-1
  aof_enabled:0
  aof_rewrite_in_progress:0
  aof_rewrite_scheduled:0
  aof_last_rewrite_time_sec:-1
  aof_current_rewrite_time_sec:-1
  aof_last_bgrewrite_status:ok
  
  # Stats
  total_connections_received:3
  total_commands_processed:8
  instantaneous_ops_per_sec:0
  rejected_connections:0
  expired_keys:0
  evicted_keys:0
  keyspace_hits:0
  keyspace_misses:0
  pubsub_channels:0
  pubsub_patterns:0
  latest_fork_usec:0
  
  # Replication
  role:master
  connected_slaves:0
  
  # CPU
  used_cpu_sys:18577.25
  used_cpu_user:1376055.38
  used_cpu_sys_children:0.00
  used_cpu_user_children:0.00
  
  # Keyspace
  redis 127.0.0.1:6380&gt;
  </code></pre>