Aof & RDB of J2 redis

AOF

Do not block the current command , But it will block the next command , Downtime before logging There is a possibility of loss

Three writeback strategies

AOF appendfsync Three strategies of

• Always, Every time the command is written, it is executed , Write disk immediately （ Basically no data loss , Affect performance ）
• Everysec, Write back every second , First write the log to AOF File buffer , Write to disk every second （ One second of data may be missing ）
• No Finish the write operation , Put the log into the buffer , The operating system decides when to write （ Good performance , The possibility of losing data is great ）
  

AOF The file will keep getting bigger , Recovery will also be slow , Need AOF Rewrite mechanism （ Save the latest data of key value pairs , There is no need to save old commands ）

• AOF Rewriting does not block the main thread , By the background thread bgrewriteaof To complete .
• a copy （ The main thread fork Out bgrewriteaof Subprocesses ）, Two logs （ At this time, you can still perform new operations , write in AOF Log and rewrite log cache , The latest operation will not be lost ）
  

RDB

• comparison AOF The recovery is relatively fast , Perform a full backup
• Provide two command generation RDB,save（ Main thread execution , It will block ） and bgsave（ Child thread write RDB file ）
• bgsave When , Through copy on write technology （copy-on-write） Can perform write operations （bgsave By main thread fork Generate , The main thread data can be shared ）
  
  Frequent full snapshots can lead to ：1. High disk pressure , It's not finished yet Later, I began to do ,2. fork The process will cause blocking of the main thread

Redis 4.0 A mixed use of AOF Log and memory snapshot methods , Use... Between snapshots AOF.

• When data cannot be lost , Memory snapshot and AOF It's a good choice to use a mixture of ;
• If minute level data loss is allowed , You can just use RDB;
• If only AOF, priority of use everysec Configuration options , Because it takes a between reliability and performance
  Balance .