默认情况下，redis的最大内存是不限制的，如下：

127.0.0.1:30003> CONFIG GET maxmemory
1) "maxmemory"
2) "0"

我把它修改成了100M，然后向里面插数据：

127.0.0.1:30003> CONFIG SET maxmemory 100
OK
127.0.0.1:30003> CONFIG GET maxmemory
1) "maxmemory"
2) "100"

当内存达到接近100M的时候，出现以下错误：

(error) OOM command not allowed when used memory > 'maxmemory'.

这个时候可以理解。接下来我用FLUSHDB将数据库清空，用INFO查看：

192.168.10.15:30005> FLUSHDB
OK
192.168.10.15:30005> INFO
# Server
redis_version:3.2.6
redis_git_sha1:00000000
redis_git_dirty:0
redis_build_id:9aa069013865e216
redis_mode:cluster
os:Linux 4.4.0-51-generic x86_64
arch_bits:64
multiplexing_api:epoll
gcc_version:4.8.4
process_id:1439
run_id:06be9bf801628e5c08f280067c6be609fb3ae8d5
tcp_port:30005
uptime_in_seconds:254023
uptime_in_days:2
hz:10
lru_clock:14205482
executable:/usr/local/redis-3.2.6/redis-server
config_file:/usr/local/redis-3.2.6/cluster/30005/redis.conf

# Clients
connected_clients:9
client_longest_output_list:0
client_biggest_input_buf:0
blocked_clients:0

# Memory
used_memory:2637448
used_memory_human:2.52M
used_memory_rss:110133248
used_memory_rss_human:105.03M
used_memory_peak:319544592
used_memory_peak_human:304.74M
total_system_memory:33655275520
total_system_memory_human:31.34G
used_memory_lua:37888
used_memory_lua_human:37.00K
maxmemory:100
maxmemory_human:100B
maxmemory_policy:noeviction
mem_fragmentation_ratio:41.76
mem_allocator:jemalloc-4.0.3

# Persistence
loading:0
rdb_changes_since_last_save:1254332
rdb_bgsave_in_progress:0
rdb_last_save_time:1490353231
rdb_last_bgsave_status:ok
rdb_last_bgsave_time_sec:0
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
aof_last_write_status:ok

# Stats
total_connections_received:137
total_commands_processed:966215
instantaneous_ops_per_sec:5
total_net_input_bytes:210514819
total_net_output_bytes:21337992636
instantaneous_input_kbps:0.33
instantaneous_output_kbps:5.29
rejected_connections:0
sync_full:1
sync_partial_ok:0
sync_partial_err:0
expired_keys:0
evicted_keys:0
keyspace_hits:0
keyspace_misses:0
pubsub_channels:1
pubsub_patterns:0
latest_fork_usec:858
migrate_cached_sockets:0

# Replication
role:master
connected_slaves:1
slave0:ip=192.168.10.14,port=30004,state=online,offset=195758203,lag=1
master_repl_offset:195758203
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:194709628
repl_backlog_histlen:1048576

# CPU
used_cpu_sys:243.60
used_cpu_user:162.66
used_cpu_sys_children:0.00
used_cpu_user_children:0.00

# Cluster
cluster_enabled:1

# Keyspace

可以看到，redis使用的内存几乎清空，这个时候我再插数据：

192.168.10.15:30005> set a 1
(error) OOM command not allowed when used memory > 'maxmemory'.

仍然出现了这个错误。

后来才发现，是单位的原因，我以为CONFIG SET maxmemory 100的单位是M，实际上不是：

maxmemory:100
maxmemory_human:100B

单位其实是Byte，怪不得出现这个错误了，将大小设置成100000000之后，就没出现错误了。

maxmemory:100000000
maxmemory_human:95.37M

作者：茶褪色
来源：存在的记忆
原文：https://looaon.com/index.php/redis/817.html
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