Combined with practice, you will understand redis persistence

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So called persistence , Is to write the contents of the cache to the disk for permanent storage （ You don't delete , If the disk is not bad, it is permanent ）

RDB

RDB yes Redis Default persistence scheme .RDB snapshot （Redis DataBase）： When satisfied Under certain conditions When , Will write the data in the current memory to disk , Generate a snapshot file dump.rdb.

Paste configuration first ：

#  File path ,
dir ./
#  File name 
dbfilename dump.rdb
#  Whether it is  LZF  Compress  rdb  file 
rdbcompression yes
#  Turn on data verification  sht
rdbchecksum yes

Redis The restart will go through dump.rdb File recovery data . that Certain conditions What is? ？ When to write rdb file ？

Automatic triggering

1. Configuration rules trigger

redis.conf, SNAPSHOTTING To configure , It defines the trigger Save data to disk Trigger frequency . If you don't need to RDB programme , notes Or configure it as an empty string " " .

save 900 1 # 900  At least one in two seconds  key  Be modified （ Including adding ）
save 300 10 # 300  At least in seconds  10  individual key  Be modified 
save 60 10000 # 60  At least in seconds  10000  individual  key  Be modified 
 Note that the above configuration does not conflict , As long as any one is satisfied, it will trigger .

2. shutdown Trigger , Make sure the server is shut down , It will not cause data loss when closing

Manual trigger

If we need to restart services or migrate data , At this time, it needs to be triggered manually RDB Snapshot save . therefore redis Also provided 2 A manual save RDB Snapshot instructions .

1. Save save Blocking the current... While generating the snapshot Redis The server ,Redis Can't handle other orders . If there is more data in memory , Can cause Redis A long block . This command is not recommended for production environments . To solve this problem ,Redis Provides a second way .
2. bgsave perform bgsave when ,Redis The snapshot operation will be performed asynchronously in the background , The snapshot can also respond to client requests . The specific operation is Redis Process execution fork（ Create process functions ） Action create subprocess (copy-on-write),RDB The persistence process is the responsibility of the subprocess , It will automatically end when it is finished . It doesn't record fork The subsequent orders . The blockage only happens in fork Stage , The average time is very short . use lastsave Command to view the last time a snapshot was successfully generated .

Show it once RDB Snapshot is a process of restoring data ！（ My snapshot is src below ）

We first add a copy of the data and back it up to RDB：

redis> set k1 1
redis> set k2 2
redis> set k3 3

See if the data exists ：

redis>keys *

We carry out shutdown Operation trigger RDB snapshot ：（ Simulate accidental power failure ）

redis> shutdown

To the existing RDB Data backup cp：

redis>cp dump.rdb dump.rdb.bak

start-up redis：

redis> src/redis-server& redis.conf

The data is still there , Now simulate data loss （ Clear it by hand ）

redis> flushall

Stop and restart the server

redis> shutdown
redis> src/redis-server& redis.conf

It is found that the data has been lost , We now need to recover from our backup data , Shut down first

redis> shutdown

Delete the original RDB The backup data cd

redis>rm dump.rdb

Rename the backup data dump.rdb

mv dump.rdb.bak dump.rdb

Restart the service

src/redis-server& redis.conf

Take a look at the data by yourself （ Manual funny ）

We know RDB The principle logic of the realization of , So let's analyze RDB What are the advantages and disadvantages .

advantage

• RDB It's a very compact (compact type ) The file of , It has been saved. redis Data set at a certain point in time . This file is ideal for backup and disaster recovery .
• Generate RDB When you file ,redis The main process will fork() Reprocess a subprocess to handle all the saving work , The main process does not need any disks IO operation .
• RDB Speed ratio when recovering large data sets AOF It's faster to recover .

Inferiority

• RDB Modal data cannot be persisted in real time / Second persistence . because bgsave Run every time fork Action create subprocess , Frequent execution cost is too high .
• Make a backup at regular intervals , So if redis accident down If you drop it , All changes since the last snapshot will be lost （ Data is missing ）.

In view of the disadvantages ,redis It also provides a persistence mechanism , Namely AOF Persistence mechanism ！

AOF

AOF yes Append Only File For short , Load only files , This program is in redis Not on by default .

AOF How to recover ？

AOF Log every write operation , And add it to the file .

After opening , Make changes Redis When data is ordered , Will write the command to AOF In file .Redis During the restart, the write instructions will be executed from the front to the back according to the contents of the log file to complete the data recovery .

So where does it start , What are the characteristics ？

Or paste configuration ： The configuration file redis.conf

#  switch 
appendonly no
#  file name 
appendfilename "appendonly.aof"

Synchronization mechanism

AOF, Each write operation will be recorded , So here comes the question ？ Do I have to interact with the disk every time I operate the command ？ Of course not , therefore ,redis Support several strategies , It's up to you to decide whether you want to interact with the disk every time .

appendfsync always Indicates that every write is executed fsync( Refresh ) function
appendfsync everysec Once a second fsync function Default 1s once , At most 1s The loss of
appendfsync no Data synchronization to disk is guaranteed by the operating system , The fastest

Rewrite mechanism （rewrite）

Why rewrite ？

such as , We use distributed locks , Instruction is setnx, Then set the expiration date ; Because the command is appended every time . that 1 After year ,10 After year , We're going to find this AOF The file is full of such instructions ！！

One of me 1T Of AOF file , It's all such instructions . So we need to rewrite .

AOF File rewriting is not about rearranging the original file , Instead, read the existing key value pairs of the server directly , Then use one command to replace the previous commands recording this key value pair , Create a new file and replace the original one AOF file .

How to rewrite ：

4.0 Before the release , Is to compare instructions and remove invalid instructions

If ：aof There are several instructions in the document ：

redis> lpush huihuilist a
redis> lpush huihuilist b c d
redis> lpop huihuilist

It will be rewritten as

redis> lpush huihuilist a b c

Later it was found that , This method is inefficient , It needs a zone by zone comparison ！

therefore ,4.0 And then introduced RDB Follow AOF Mixed mode , Give Way RDB Follow AOF Use it together , because RDB Fast , So let's introduce this feature , according to RDB Pattern , Overwrite the previous instructions in binary form to aof, Those written later continue to pursue to the back of the file .（ Mixed mode is on by default , You can also turn off ）

aof-use-rdb-preamble yes // Open or not RDB And AOF Mixed mode 

When to rewrite ？

The configuration file redis.conf

#  Override trigger mechanism 
auto-aof-rewrite-percentage 100
auto-aof-rewrite-min-size 64mb 
// In the beginning aof The file must reach this file size before triggering , Each subsequent rewrite will not be based on this variable 

Explain the meaning of the above configuration ：

stay aof The file is less than 64mb Do not rewrite when , When they arrive in 64mb When , Just rewrite it once .
The rewritten aof The document could be 20mb. It's configured auto-aof-rewrite-percentag by 100, namely aof Here is the document 40mb When , Start rewriting again . And so on .

We know AOF Implementation principle of , Let's analyze its advantages and disadvantages .

advantage

• It can guarantee data security to the greatest extent , Even with the default configuration everysec, At best, it will only cause 1s Data loss for .

shortcoming

• Data volume ratio RDB It's much bigger. , So there's no performance RDB good , There is no performance guarantee ！

What kind of persistence should we use in our daily development ？

If we can tolerate the loss of data in a short period of time , There is no doubt about using RDB It's the best , Timed generation RDB snapshot , Very convenient for database backup , also RDB Data set recovery is also faster than AOF Fast recovery .

author ： Xiao Zhao, who is striving for millions of annual salary

https://blog.csdn.net/weixin_44688973/article/details/125335060

PS： If the article helps , give the thumbs-up 、 Looking at 、 Forward! ！

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