问题二：如何判断master和slave复制的延迟是多少？

问题二：

In my production , the slaves replication is always lag for many hours!
127.0.0.1:6379> info replication

Replication
role:master
connected_slaves:2
slave0:ip=10.xxx.xxx.xxx,port=6379,state=online,offset=416543935501,lag=1
slave1:ip=10.xxx.xxx.xxx,port=6379,state=online,offset=416543965574,lag=1
master_repl_offset:416543969598
repl_backlog_active:1
repl_backlog_size:1048576
repl_backlog_first_byte_offset:416542921023
repl_backlog_histlen:1048576

the offset of slave* and master_repl_offset are big different.
how to solve or optimize the problem?

答案：

The values here all make sense, they just aren't clearly explained and have some weird names.

When you see repl_backlog those are only for PSYNC. So, they could probably be named psync_buffer for the same effect.

repl_backlog_size is the capacity of a buffer holding data for PSYNC. repl_backlog_histlen is how much actual data is in the PSYNC buffer. They will usually be equal since repl_backlog_histlen can only grow as big as repl_backlog_size.

Also notice how the backlog first byte offset (repl_backlog_first_byte_offset) is equal to the maximum PSYNC buffer size (repl_backlog_size) which is also equal to the currently populated PSYNC buffer data (repl_backlog_histlen). So, master_repl_offset - repl_backlog_first_byte_offset = repl_backlog_size: 416543969598 - 416542921023 = 1048575 (yeah, there's an off-by-one error somewhere).

The actual lag is the difference between each slave offset and the master_repl_offset. So, in this case, slave0 is 416543969598 - 416543935501 = 34 KB behind the master and slave1 is 416543969598 - 416543965574 = 4 KB behind the master.

The actual replication lag could be reported nicer in the INFO output, but... it isn't. :-\

repl_backlog开头的参数都是针对PSYNC的，应该叫做psync_buffer更合适。
repl_backlog_size是PSYNC buffer的capcacity
repl_backlog_histlen是实际PSYNC buffer中的数据量
repl_backlog_histlen能增长到和repl_backlog_size一样大
另外，master_repl_offset减去repl_backlog_first_byte_offset等于repl_backlog_histlen

实际数据延迟，master_repl_offset减去slave的offset就是该slave的数据延迟，单位是byte

时间延迟，就是lag，但是是秒（实际上这个lag 是通过master 的当前时间减去slave 通过ACK上报上来的时间得到的

参考文章：
https://github.com/antirez/redis/issues/2375

https://blog.csdn.net/mysqldba23/article/details/68066322

https://www.cnblogs.com/svan/p/7366397.html