GPDB · 特性分析 · Segment 修复指南
最后更新于：2022-04-01 10:35:28
## 问题背景
GPDB是中央控制节点式的架构，在一个 GreenPlum 集群中，有一个 Master 节点和多个 Segment 节点。Master 是中央控制节点，Segment 是数据存放节点。所有的Segment节点平等，均由Master管理。架构如下图：
GreenPlum架构图
当GP Master出现问题的时候，可以通过外部的HA监控模块发现并激活备库，Standby Master 正常后删除原来的 Master 进行重建备库。
而 Segment 的修复与此不同！由上图可知，Segment 也分为主备，称为 Primary 和 Mirror，Mirror 是 Primary 的备。Primary与Mirror之间强同步保证数据一致性和可靠性，其间的监控与切换则由Master的FTS模块负责。当FTS发现Primary宕机、Mirror健康后会激活Mirror，并标记Primary为’d’，Mirror进入 ChangeTracking 状态。(详细的原理此处不作赘述，有兴趣可以参考本期月报的[GPDB · 特性分析· GreenPlum Segment事务一致性与异常处理](http://mysql.taobao.org/monthly/2016/04/02/)和上期的[GPDB · 特性分析· GreenPlum FTS 机制](http://mysql.taobao.org/monthly/2016/03/08/))
当有Segment被标记为’d’后，Master将不会对其做处理，GP实例的启动（重启）也会将其忽略。这个时候，整个GP集群是处于有风险的状况中：
1. 切过去的Mirror压力增大（需要做change tracking）；
2. 节点单点，可靠性风险加大。
这个时候需要及时地对Segment进行修复。
## GP的Segment修复
GP提供了一系列的控制脚本用于对GP进行操作，其中用于修复Segment的是gprecoverseg。使用方式比较简单，有限的几个主要参数如下：
* -i
主要参数，用于指定一个配置文件，该配置文件描述了需要修复的Segment和修复后的目的位置。
* -F
可选项，指定后，gprecoverseg会将”-i”中指定的或标记”d”的实例删除，并从活着的Mirror复制一个完整一份到目标位置。
* -r
当FTS发现有Primary宕机并进行主备切换，在gprecoverseg修复后，担当Primary的Mirror角色并不会立即切换回来，就会导致部分主机上活跃的Segment过多从而引起性能瓶颈。因此需要恢复Segment原先的角色，称为re-balance。
举个使用的例子：
下面是一个正常的实例，
~~~
$ gpstate -s
/opt/python27/lib/python2.7/site-packages/Crypto/Util/number.py:57: PowmInsecureWarning: Not using mpz_powm_sec. You should rebuild using libgmp >= 5 to avoid timing attack vulnerability.
_warn("Not using mpz_powm_sec. You should rebuild using libgmp >= 5 to avoid timing attack vulnerability.", PowmInsecureWarning)
20160418:21:39:29:016547 gpstate:host1:gpuser-[INFO]:-Starting gpstate with args: -s
20160418:21:39:29:016547 gpstate:host1:gpuser-[INFO]:-local Greenplum Version: 'postgres (Greenplum Database) 4.3.99.00 build dev'
20160418:21:39:29:016547 gpstate:host1:gpuser-[INFO]:-master Greenplum Version: 'PostgreSQL 8.3 (Greenplum Database 4.3.99.00 build dev) compiled on Apr 11 2016 22:02:39'
20160418:21:39:29:016547 gpstate:host1:gpuser-[INFO]:-Obtaining Segment details from master...
20160418:21:39:29:016547 gpstate:host1:gpuser-[INFO]:-Gathering data from segments...
.
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:-----------------------------------------------------
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:--Master Configuration & Status
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:-----------------------------------------------------
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Master host = host1
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Master postgres process ID = 72447
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Master data directory = /workspace/gpuser/3007
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Master port = 3007
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Master current role = dispatch
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Greenplum initsystem version = 4.3.99.00 build dev
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Greenplum current version = PostgreSQL 8.3 (Greenplum Database 4.3.99.00 build dev) compiled on Apr 11 2016 22:02:39
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Postgres version = 8.3
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Master standby = host2
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Standby master state = Standby host passive
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:-----------------------------------------------------
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:-Segment Instance Status Report
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:-----------------------------------------------------
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Segment Info
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Hostname = host1
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Address = host1
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Datadir = /workspace/gpuser/3008
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Port = 3008
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Mirroring Info
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Current role = Primary
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Preferred role = Primary
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Mirror status = Synchronized
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Status
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- PID = 72388
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Configuration reports status as = Up
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Database status = Up
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:-----------------------------------------------------
......
[INFO]:-----------------------------------------------------
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Segment Info
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Hostname = host1
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Address = host1
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Datadir = /workspace/gpuser/3012
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Port = 3012
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Mirroring Info
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Current role = Mirror
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Preferred role = Mirror
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Mirror status = Synchronized
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Status
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- PID = 75247
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Configuration reports status as = Up
20160418:21:39:30:016547 gpstate:host1:gpuser-[INFO]:- Segment status = Up
~~~
选择一个kill之后（如3012这个端口的实例），执行gprecoverseg，如下：
~~~
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3008
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3008
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3014
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3014
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3010
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3010
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3015
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3015
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3008
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3008
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3011
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3011
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3013
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3013
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3012
20160418:21:40:58:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3012
......
20160418:21:41:18:017989 gpstate:host1:gpuser-[DEBUG]:-[worker6] finished cmd: Get segment status cmdStr='sshpass -e ssh -o 'StrictHostKeyChecking no' host1 ". /workspace/gpdb/greenplum_path.sh; $GPHOME/bin/gp_primarymirror -h host1 -p 3012"' had result: cmd had rc=15 completed=True halted=False
stdout=''
stderr='failed to connect: Connection refused (errno: 111)
Retrying no 1
failed to connect: Connection refused (errno: 111)
Retrying no 2
......
20160418:21:41:18:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Encountered error Not ready to connect to database mode: PrimarySegment
segmentState: Fault
dataState: InSync
faultType: FaultMirror
mode: PrimarySegment
segmentState: Fault
dataState: InSync
faultType: FaultMirror
~~~
这个时候连接这个实例去获取信息是失败的，失败的原因后面再说。这个时候失败后会重试5次，当再一次尝试的时候发现了不同：
~~~
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3008
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3008
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3014
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3014
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3010
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3010
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3015
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3015
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3008
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3008
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host1 -p 3011
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host1 -p 3011
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Sending msg getStatus and cmdStr $GPHOME/bin/gp_primarymirror -h host2 -p 3013
20160418:21:41:23:017989 gprecoverseg:host1:gpuser-[DEBUG]:-Adding cmd to work_queue: $GPHOME/bin/gp_primarymirror -h host2 -p 3013
~~~
会发现，少了一个Segment的命令，而这个Segment正是刚才kill的Segment。继续往下看执行结果，gprecoverseg执行了下面的內容：
~~~
20160418:23:16:20:085203 gprecoverseg:host1:gpuser-[DEBUG]:-[worker7] finished cmd: Get segment status information cmdStr='sshpass -e ssh -o 'StrictHostKeyChecking no' host2 ". /workspace/gpdb/greenplum_path.sh; $GPHOME/bin/gp_primarymirror -h host2 -p 3013"' had result: cmd had rc=1 completed=True halted=False
stdout=''
stderr='mode: PrimarySegment
segmentState: Ready
dataState: InChangeTracking
faultType: NotInitialized
mode: PrimarySegment
segmentState: Ready
dataState: InChangeTracking
faultType: NotInitialized
'
~~~
这个实例为什么单独检查呢？而且这个时候如果失败，则会直接退出无法继续执行。
在一系列的检查之后，先更新catalog中的操作记录表：
~~~
UPDATE pg_catalog.gp_segment_configuration
~~~
之后即调用命令进行数据的恢复：
~~~
/workspace/gpdb/bin/lib/gpconfigurenewsegment -c /workspace/gpuser/3012:3012:false:false:9 -v -B 16 --write-gpid-file-only
~~~
最后再启动Segment，并更新catalog：
~~~
$GPHOME/sbin/gpsegstart.py -C en_US.utf8:C:C -M quiescent -V 'postgres (Greenplum Database) 4.3.99.00 build dev' -n 4 --era df86ca11ca2fc214_160418165251 -t 600 -v -p KGRwMApTJ2Ric0J5UG9ydCcKcDEKKGRwMgpJMzAxMgooZHAzClMndGFyZ2V0TW9kZScKcDQKUydtaXJyb3InCnA1CnNTJ2RiaWQnCnA2Ckk5CnNTJ2hvc3ROYW1lJwpwNwpTJzEwLjk3LjI0OC43MycKcDgKc1MncGVlclBvcnQnCnA5CkkzNTEzCnNTJ3BlZXJQTVBvcnQnCnAxMApJMzAxMwpzUydwZWVyTmFtZScKcDExClMncnQxYjA3MDI0LnRiYycKcDEyCnNTJ2Z1bGxSZXN5bmNGbGFnJwpwMTMKSTAwCnNTJ21vZGUnCnAxNApTJ3InCnAxNQpzUydob3N0UG9ydCcKcDE2CkkzNTEyCnNzcy4= -D '9|3|m|m|r|d|host1|host1|3012|3512|/workspace/gpuser/3012||'
......
20160419:01:21:05:042692 gprecoverseg:host1:gpuser-[DEBUG]:-UPDATE pg_catalog.gp_segment_configuration
SET
mode = 'r',
status = 'u'
WHERE dbid = 5
20160419:01:21:05:042692 gprecoverseg:host1:gpuser-[DEBUG]:-INSERT INTO gp_configuration_history (time, dbid, "desc") VALUES(
now(),
5,
'gprecoverseg: segment resync marking mirrors up and primaries resync: segment mode and status'
)
20160419:01:21:05:042692 gprecoverseg:host1:gpuser-[DEBUG]:-UPDATE pg_catalog.gp_segment_configuration
SET
mode = 'r',
status = 'u'
WHERE dbid = 9
20160419:01:21:05:042692 gprecoverseg:host1:gpuser-[DEBUG]:-INSERT INTO gp_configuration_history (time, dbid, "desc") VALUES(
now(),
9,
'gprecoverseg: segment resync marking mirrors up and primaries resync: segment mode and status'
)
20160419:01:21:05:042692 gprecoverseg:host1:gpuser-[DEBUG]:-UPDATE gp_fault_strategy
~~~
这样即是一个完整的gprecoverseg过程。执行过后，对应的Primary和Mirror会进入”r”状态，表示正在做数据同步。
下面来看其中的详细步骤和原理。
## 实现原理
上面的例子中，遗留了几个问题：
* 在gprecoverseg过程中，第一次获取Segment状态是不对的；
* 第二次获取Segment信息，比第一次少了一条；
* 单独检查了“-h host2 -p 3013”这个实例。
这几个问题在了解了原理后就很容易理解了。想要了解原理，可以先看下执行的步骤。从代码看来，其大致的步骤如下：
### 参数处理
GP的脚本用了较多的环境变量，且不同的脚本、不同的地方略有不同。如gprecoverseg用的就是MASTER_DATA_DIRECTORY，从MASTER_DATA_DIRECTORY指定的目录中得到Master相关的信息（如port）以进行相关操作。
gprecoverseg的参数，最重要的莫过于”-i”了，其指定了需要做修复的Segment，并且可以指定到不同的主机上，例如：
~~~
filespaceOrder=
host1:3012:/workspace/gpuser/3012 host2:3012:3512:/workspace/gpuser/3012
~~~
具体执行不再赘述。
### 判断Segment当前的状态
调用gp_primarymirror，向活着的segment发送消息，以判断Segment当前的状态。这是非常重要的一步，也是遇到问题最多的一步，经常会出现问题”Unable to connect to database”。事实上，造成这个失败的原因有很多，比较多的是：
* 其对应的Primary（Mirror）也宕机；
* 其对应的Primary的状态不对，如已经有gprecoverseg在进行（或执行失败，状态出问题等）。
在做这一步的时候，是依赖gp_segment_configuration中的数据的，即会首先从GP Master上获取相应的数据，与下一步中的描述基本相同。
如果这个Segment被标记为”d”，那么是不会向该Segment发起状态信息请求。
而如果对应的Primary/Mirror都宕机了，他们的状态不会同时为”d”（有可能都为”u”，比如同时异常的时候，FTS不会更新他们）。因此对标记为”u”实际已经宕机的Segment连接获取状态信息的时候，则会报错。这个时候就不是gprecoverseg所能处理的问题了，只能重启整个实例。
回到前面的问题。第一次执行失败即因为Segment的状态尚未更新；第二次执行少了一个Segment，即状态被更新为”d”后不进行连接。
在检查完所有状态为”u”的Segment连接后，则会针对宕机的Mirror进行检查，查看其对应的主库是否正常，可以用于修复数据，即是第三个问题的答案。如：
~~~
stdout=''
stderr='mode: PrimarySegment
segmentState: Ready
dataState: InSync
faultType: NotInitialized
mode: PrimarySegment
segmentState: Ready
dataState: InSync
faultType: NotInitialized
'
~~~
或者这样
~~~
stdout=''
stderr='mode: PrimarySegment
segmentState: Ready
dataState: InChangeTracking
faultType: NotInitialized
mode: PrimarySegment
segmentState: Ready
dataState: InChangeTracking
faultType: NotInitialized
'
~~~
正常情况下，当Mirror出现问题，Primary发现后会进入ChangeTracking的状态。在这个状态里，Primary会记录下切换状态时间点之后的变更，用于当Mirror恢复时进行数据同步，而不用每次都做一次全量。
### 从master获取segment的信息
包括IP、PORT、ROLE、Status、数据目录、临时空间等，如下：
~~~
dbid | content | role | preferred\_role | mode | status | hostname | address | port | replication\_port | oid | fselocation
------+---------+------+----------------+------+--------+---------------+---------------+------+------------------+------+-----------------
1 | -1 | p | p | s | u | host1 | host1 | 3007 | | 3052 | /workspace/gpuser/3007
10 | -1 | m | m | s | u | host2 | host2 | 3007 | | 3052 | /workspace/gpuser/3007
2 | 0 | p | p | s | u | host1 | host1 | 3008 | 3508 | 3052 | /workspace/gpuser/3008
6 | 0 | m | m | s | u | host2 | host2 | 3014 | 3514 | 3052 | /workspace/gpuser/3014
3 | 1 | p | p | s | u | host1 | host1 | 3010 | 3510 | 3052 | /workspace/gpuser/3010
7 | 1 | m | m | s | u | host2 | host2 | 3015 | 3515 | 3052 | /workspace/gpuser/3015
4 | 2 | p | p | s | u | host2 | host2 | 3008 | 3508 | 3052 | /workspace/gpuser/3008
8 | 2 | m | m | s | u | host1 | host1 | 3011 | 3511 | 3052 | /workspace/gpuser/3011
5 | 3 | p | p | s | u | host2 | host2 | 3013 | 3513 | 3052 | /workspace/gpuser/3013
9 | 3 | m | m | s | u | host1 | host1 | 3012 | 3512 | 3052 | /workspace/gpuser/3012
~~~
IP/PORT/ROLE/STATUS/目录/FILESPACE等信息，后面的Mirror修复列表、临时空间、操作对象的信息都依赖于此。
### 修复准备
在获取所有的Segment信息后，会针对配置文件、参数等相关信息确定，包括：
* Segment修复对象
确定Segment修复对象和数据源，即Primary；需要修复的Segment有可能是多个。并获取需要修复的Segment的相关信息，包括端口、流复制端口、数据目录、临时空间、文件空间等信息，以及是否强制修复等。
* 主机环境
在获取所需要修复的Segment列表后，需要确保所在主机环境是可以的，包括端口占用、目录的占用等有可能冲突的地方。
如果没有指定主机，则会在已有的主机中选择一个。
### 修复
修复的步骤是：
* 关闭宕机的Mirror，并清理shared memory
* 确定需要修复的Segment已经被标记为”d”
* 如有需要，则进行删除，如”-F”的情况
* 打包压缩、复制数据到目标位置
* 关闭SIGINT、SIG_IGN，更新元数据库，打开SIGINT、SIG_IGN
以上步骤后，即可实现对Segment的本地（in-place）或跨机修复。
### re-balance
当修复完Segment之后，原先因为Primary宕机而切到Mirror上的Segment并不会主动切回来，这个时候有可能出现性能倾斜而影响性能，因此需要做”re-balance”，执行：
~~~
gprecoverseg -r
~~~
执行该命令会将role切换为preferred_role，保证整个集群的角色平衡而不致于部分主机Primay更多引起性能瓶颈。