Alibaba Cloud has developed ApsaraDB for Redis 4.0 based on Redis 4.0. ApsaraDB for Redis 4.0 supports a variety of new features and fixes several issues. Redis 4.0 provides all the benefits of the Redis 2.8 engine and provides the following features.
Redis 4.0 supports the Lazyfree feature. This feature can avoid congestion on the
Redis-server caused by the
RENAME commands and ensure service stability. The following sections describes this feature.
For versions that are earlier than Redis 4.0,
OK is returned only after all memory resources of a key are released when the
DEL command is run. If the key contains a large amount of data, such as 10 million entries
in a hash table, other connections may remain pending for a long time period. To be
compatible with the existing
DEL syntax, Redis 4.0 uses the
UNLINK command. The UNLINK command can be used in the same way that you use the
DEL command. However, the background thread releases the memory when Redis 4.0 runs the
UNLINK key [key ...]
FLUSHDB and FLUSHALL commands in Redis 4.0 allow you to specify whether to use the Lazyfree feature to
release the memory.
FLUSHALL [ASYNC] FLUSHDB [ASYNC]
When you run the
RENAME OLDKEY NEWKEY command, if the specified new key exists, Redis first deletes the existing new key.
If the key contains a large amount of data, other connections remain pending for a
long time period. To use the Lazyfree feature to delete the key, set the following
configuration in the console:
This parameter is not available in the console.
Expire or evict data
You can specify data expiration time and allow Redis to delete expired data. However, CPU jitter may occur when a large expired key is deleted. Redis 4.0 allows you to specify whether to use the Lazyfree feature to expire or evict data.
lazyfree-lazy-eviction yes/no lazyfree-lazy-expire yes/no
SWAPDB command is used to swap two Redis databases. After you run the
SWAPDB command, you can query the new data from the other database without executing the
127.0.0.1:6379> select 0 OK 127.0.0.1:6379> set key value0 OK 127.0.0.1:6379> select 1 OK 127.0.0.1:6379> set key value1 OK 127.0.0.1:6379> swapdb 0 1 OK 127.0.0.1:6379> get key "value0" 127.0.0.1:6379> select 0 OK 127.0.0.1:6379> get key "value1"
ZLEXCOUNT command is used to return the number of elements in a sorted set. The
ZLEXCOUNT command is similar to the
ZRANGEBYLEX command. However, the
ZRANGEBYLEX command returns all the elements in the sorted set.
Versions that are earlier than Redis 4.0 use the
INFO MEMORY command to provide the memory information. Redis 4.0 allows you to run the
MEMORY command to obtain further information about the memory status.
127.0.0.1:6379> memory help 1) "MEMORY DOCTOR - Outputs memory problems report" 2) "MEMORY USAGE <key> [SAMPLES <count>] - Estimate memory usage of key" 3) "MEMORY STATS - Show memory usage details" 4) "MEMORY PURGE - Ask the allocator to release memory" 5) "MEMORY MALLOC-STATS - Show allocator internal stats"
USAGEchild command is used to view the memory usage of a specified key in Redis.Notice
- Key-value pairs in Redis consume memory. Redis also consumes memory to manage data.
- The memory usage of keys such as hash tables, lists, sets, and sorted sets is calculated
based on sampling. The
SAMPLEScommand is used to specify the number of samples.
126.96.36.199:6379> memory stats 1) "peak.allocated" // The maximum memory that Redis used after startup. 2) (integer) 423995952 3) "total.allocated" //The current memory usage. 4) (integer) 11130320 5) "startup.allocated" //The memory that Redis uses after startup and initialization. 6) (integer) 9942928 7) "replication.backlog" //The amount of memory of the backlog used in resuming an interrupted master-replica replication. Default value: 10 MB. 8) (integer) 1048576 9) "clients.slaves" // The amount of memory used in a master-replica replication. 10) (integer) 16858 11) "clients.normal" //The amount of memory used by read and write buffers for common clients. 12) (integer) 49630 13) "aof.buffer" //The sum of the cache used for append-only file (AOF) persistence and the cache generated during the AOF rewrite operations. 14) (integer) 3253 15) "db.0" //The memory used by metadata in each database. 16) 1) "overhead.hashtable.main" 2) (integer) 5808 3) "overhead.hashtable.expires" //The memory used to manage the data that has TTL configured. 4) (integer) 104 17) "overhead.total" //The total used memory for the preceding items. 18) (integer) 11063904 19) "keys.count" //The total number of keys in the current storage. 20) (integer) 94 21) "keys.bytes-per-key" //The average size of each key in the current memory. 22) (integer) 12631 23) "dataset.bytes" //The memory used by user data (= Total memory - Memory used by Redis metadata). 24) (integer) 66416 25) "dataset.percentage" //100 * dataset.bytes / (total.allocated - startup.allocated) 26) "5.5934348106384277" 27) "peak.percentage" // 100 * total.allocated / peak_allocated 28) "2.6251003742218018" 29) "fragmentation" //The memory fragmentation ratio. 30) "1.1039986610412598"
This command is used to provide diagnostics and identify potential issues.
Peak memory: peak.allocated/total.allocated > 1.5. This indicates that the memory fragmentation ratio may be high. High fragmentation: fragmentation > 1.4. This indicates a high memory fragmentation ratio. Big slave buffers: the average memory for each replica buffer is more than 10 MB. This may be caused by high traffic of write operations on the master node. Big client buffers: the average memory for a common client buffer is more than 200 KB. This may be caused by the improper use of a pipeline or caused by Pub/Sub clients that delay processing messages.
Both commands take effect only when you use jemalloc.
Least Frequently Used (LFU) mechanism and hotkeys
Redis 4.0 supports allkey-lfu and volatile-lfu data eviction policies, and allows
you to run the
OBJECT command to obtain the frequency of use for a specific key.
object freq user_key
Based on the LFU mechanism, you can run the
SCAN and OBJECT FREQ commands to find hotkeys. You can also use the Redis command-line interface (redis-cli)
as shown in the following example.
$./redis-cli --hotkeys # Scanning the entire keyspace to find hot keys as well as # average sizes per key type. You can use -i 0.1 to sleep 0.1 sec # per 100 SCAN commands (not usually needed). [00.00%] Hot key 'counter:000000000002' found so far with counter 87 [00.00%] Hot key 'key:000000000001' found so far with counter 254 [00.00%] Hot key 'mylist' found so far with counter 107 [00.00%] Hot key 'key:000000000000' found so far with counter 254 [45.45%] Hot key 'counter:000000000001' found so far with counter 87 [45.45%] Hot key 'key:000000000002' found so far with counter 254 [45.45%] Hot key 'myset' found so far with counter 64 [45.45%] Hot key 'counter:000000000000' found so far with counter 93 -------- summary ------- Sampled 22 keys in the keyspace! hot key found with counter: 254 keyname: key:000000000001 hot key found with counter: 254 keyname: key:000000000000 hot key found with counter: 254 keyname: key:000000000002 hot key found with counter: 107 keyname: mylist hot key found with counter: 93 keyname: counter:000000000000 hot key found with counter: 87 keyname: counter:000000000002 hot key found with counter: 87 keyname: counter:000000000001 hot key found with counter: 64 keyname: myset