redis.conf文件中可以看到slave-serve-stale-data这个参数，作用是什么？

原文解释：

# When a slave loses its connection with the master, or when the replication
# is still in progress, the slave can act in two different ways:
#
# 1) if slave-serve-stale-data is set to 'yes' (the default) the slave will
#    still reply to client requests, possibly with out of date data, or the
#    data set may just be empty if this is the first synchronization.
#
# 2) if slave-serve-stale-data is set to 'no' the slave will reply with
#    an error "SYNC with master in progress" to all the kind of commands
#    but to INFO and SLAVEOF.
#
slave-serve-stale-data yes

slave-serve-stale-data参数设置成yes，主从复制中，从服务器可以响应客户端请求；

slave-serve-stale-data参数设置成no，主从复制中，从服务器将阻塞所有请求，有客户端请求时返回“SYNC with master in progress”；