Centos7在出现系统异常的情况下，比如iscsi软件栈出错且有持续IO的情况下，很可能会出现持续大量的日志，就像短时强风暴一样，甚至导致日志丢失。
在调试阶段，这些日志都是需要的，为此需要解决此时出现的log drop。方法是修改/etc/rsyslog.conf，加入下面的几行：
$SystemLogRateLimitInterval 0
$SystemLogRateLimitBurst 0
$IMUXSockRateLimitInterval 0
$IMJournalRatelimitInterval 0
"/etc/rsyslog.conf" 106L,3683C
此外，我们还要求日志能及时落盘，为此也需要修改journald配置文件/etc/systemd/journald.conf,更新下面几行：
Storage=persistent
RateLimitInterval=0
RateLimitBurst=0
SyncIntervalSec=2
除此之外，还需要disable 日志盘的写缓存：
[root@192.168.1.84:~]$ hdparm -W 0 /dev/sda
/dev/sda:
setting drive write-caching to 0 (off)
write-caching = 0 (off)
最后重启服务：
systemctl daemon-reload
[root@localhost etc]# systemctl restart systemd-journald.service
[root@localhost etc]# systemctl restart rsyslog.service
或者重启机器就可生效。在本人的机器上，重启多次之前kernel出错及不少日志丢失的问题，根据上面的改动都被解决了。
参考：
1. man journald.conf
RateLimitInterval=,RateLimitBurst=
Configures the rate limiting that is applied to all messages generated on the system. If,in the
time interval defined by RateLimitInterval=,more messages than specified in RateLimitBurst= are
logged by a service,all further messages within the interval are dropped until the interval is
over. A message about the number of dropped messages is generated. This rate limiting is applied
per-service,so that two services which log do not interfere with each other's limits. Defaults to
1000 messages in 30s. The time specification for RateLimitInterval= may be specified in the
following units: "s","min","h","ms","us". To turn off any kind of rate limiting,set either
value to 0.
SyncIntervalSec=
The timeout before synchronizing journal files to disk. After syncing,journal files are placed in
the OFFLINE state. Note that syncing is unconditionally done immediately after a log message of
priority CRIT,ALERT or EMERG has been logged. This setting hence applies only to messages of the
levels ERR,WARNING,NOTICE,INFO,DEBUG. The default timeout is 5 minutes.
2. man journalctl
