当忘记TiDB root 密码时，可以通过设置skip-grant-table参数来跳过密码验证，登录成功以后再修改root密码。
方法一：
修改tidb-server的配置文件
vi /tidb-deploy/tidb-4000/conf/tidb.toml
# WARNING: This file is auto-generated. Do not edit! All your modification will be overwritten!
# You can use 'tiup cluster edit-config' and 'tiup cluster reload' to update the configuration
# All configuration items you want to change can be added to:
# server_configs:
# tidb:
# aa.b1.c3: value
# aa.b2.c4: value
[security]
skip-grant-table = true
~
直接修改配置文件需要重启tidb-server服务才能生效 连接到中控机，使用tiup关闭该节点的tidb-server服务
[root@node1 ~]# tiup cluster stop tidb-test -N 192.168.233.82:4000
tiup is checking updates for component cluster ...
Starting component `cluster`: /root/.tiup/components/cluster/v1.9.1/tiup-cluster /root/.tiup/components/cluster/v1.9.1/tiup-cluster stop tidb-test -N 192.168.233.82:4000
Will stop the cluster tidb-test with nodes: 192.168.233.82:4000, roles: .
Do you want to continue? [y/N]:(default=N) y
+ [ Serial ] - SSHKeySet: privateKey=/root/.tiup/storage/cluster/clusters/tidb-test/ssh/id_rsa, publicKey=/root/.tiup/storage/cluster/clusters/tidb-test/ssh/id_rsa.pub
....
+ [ Serial ] - StopCluster
Stopping component tidb
Stopping instance 192.168.233.82
Stop tidb 192.168.233.82:4000 success
Stopping component node_exporter
Stopping component blackbox_exporter
Stopped cluster `tidb-test` successfully
由于添加了skip-grant-table = true 无法直接通过tiup来启动192.168.233.82上的 tidb-server
[root@node1 ~]# tiup cluster start tidb-test -N 192.168.233.82:4000
tiup is checking updates for component cluster ...
Starting component `cluster`: /root/.tiup/components/cluster/v1.9.1/tiup-cluster /root/.tiup/components/cluster/v1.9.1/tiup-cluster start tidb-test -N 192.168.233.82:4000
Starting cluster tidb-test...
+ [ Serial ] - SSHKeySet: privateKey=/root/.tiup/storage/cluster/clusters/tidb-test/ssh/id_rsa, publicKey=/root/.tiup/storage/cluster/clusters/tidb-test/ssh/id_rsa.pub
....
+ [ Serial ] - StartCluster
Starting component tidb
Starting instance 192.168.233.82:4000
Error: failed to start tidb: failed to start: 192.168.233.82 tidb-4000.service, please check the instance's log(/tidb-deploy/tidb-4000/log) for more detail.: timed out waiting for port 4000 to be started after 2m0s
Verbose debug logs has been written to /root/.tiup/logs/tiup-cluster-debug-2022-03-05-07-50-08.log.
官方文档说明：设置 skip-grant-table 之后，启动 TiDB 进程会增加操作系统用户检查，只有操作系统的 root 用户才能启动 TiDB 进程。
查看tidb-server的日志
[root@node2 log]# tailf -n 10 tidb_stderr.log
load config file: /tidb-deploy/tidb-4000/conf/tidb.toml
invalid config TiDB run with skip-grant-table need root privilege
load config file: /tidb-deploy/tidb-4000/conf/tidb.toml
invalid config TiDB run with skip-grant-table need root privilege
load config file: /tidb-deploy/tidb-4000/conf/tidb.toml
invalid config TiDB run with skip-grant-table need root privilege
load config file: /tidb-deploy/tidb-4000/conf/tidb.toml
invalid config TiDB run with skip-grant-table need root privilege
load config file: /tidb-deploy/tidb-4000/conf/tidb.toml
invalid config TiDB run with skip-grant-table need root privilege
连接到tidb-server所在的服务器，手工执行启动脚本
/tidb-deploy/tidb-4000/scripts/run_tidb.sh
再打开一个新会话，就可以免密登录了
[root@node2 ~]# mysql -h 192.168.233.82 -P 4000 -uroot
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MySQL connection id is 5
Server version: 5.7.25-TiDB-v5.0.0 TiDB Server (Apache License 2.0) Community Edition, MySQL 5.7 compatible
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MySQL [(none)]>
为一个已存在的账户修改密码，可以通过 SET PASSWORD FOR 或者 ALTER USER 语句完成：
SET PASSWORD FOR 'root'@'%' = 'xxx';
或者：
ALTER USER 'test'@'localhost' IDENTIFIED BY 'mypass';
修改成功以后，在执行启动脚本的会话里执行ctrl+c,然后删除添加的参数
再使用tiup重新启动tidb-server
[root@node1 ~]# tiup cluster start tidb-test -N 192.168.233.82:4000
tiup is checking updates for component cluster ...
Starting component `cluster`: /root/.tiup/components/cluster/v1.9.1/tiup-cluster /root/.tiup/components/cluster/v1.9.1/tiup-cluster start tidb-test -N 192.168.233.82:4000
Starting cluster tidb-test...
+ [ Serial ] - SSHKeySet: privateKey=/root/.tiup/storage/cluster/clusters/tidb-test/ssh/id_rsa, publicKey=/root/.tiup/storage/cluster/clusters/tidb-test/ssh/id_rsa.pub
.....
+ [ Serial ] - StartCluster
Starting component tidb
Starting instance 192.168.233.82:4000
Start instance 192.168.233.82:4000 success
Starting component node_exporter
Starting instance 192.168.233.82
Start 192.168.233.82 success
Starting component blackbox_exporter
Starting instance 192.168.233.82
Start 192.168.233.82 success
+ [ Serial ] - UpdateTopology: cluster=tidb-test
Started cluster `tidb-test` successfully
因为密码信息是存储再TiKV中的，所以当密码修改成功以后，所有的TiDB-server节点都可以使用新密码登录
方法二：
使用tiup 来修改配置文件
tiup cluster edit-config tidb-test
global:
user: tidb
ssh_port: 22
ssh_type: builtin
deploy_dir: /tidb-deploy
data_dir: /tidb-data
os: linux
arch: amd64
server_configs:
tidb:
security.skip-grant-table: true
monitored:
node_exporter_port: 9100
blackbox_exporter_port: 9115
deploy_dir: /tidb-deploy/monitor-9100
下面是添加的内容（注意折行）
server_configs:
tidb:
security.skip-grant-table: true
添加完成以后，需要执行reload语句来同步到节点上去。
Please check change highlight above, do you want to apply the change? [y/N]:(default=N) y
Applying changes...
Applied successfully, please use `tiup cluster reload tidb-test [-N <nodes>] [-R <roles>]` to reload config.
[root@node1 ~]# tiup cluster reload tidb-test
- Generate config pd -> 192.168.233.83:2379 ... Done
- Generate config tikv -> 192.168.233.81:20160 ... Done
- Generate config tikv -> 192.168.233.82:20160 ... Done
- Generate config tikv -> 192.168.233.83:20160 ... Done
- Generate config tidb -> 192.168.233.82:4000 ... Error
- Generate config tidb -> 192.168.233.83:4000 ... Error
- Generate config prometheus -> 192.168.233.81:9090 ... Done
- Generate config grafana -> 192.168.233.81:3000 ... Done
- Generate config alertmanager -> 192.168.233.81:9093 ... Done
Error: init config failed: 192.168.233.83:4000: executor.ssh.execute_failed: Failed to execute command over SSH for 'tidb@192.168.233.83:22' {ssh_stderr: load config file: /tidb-deploy/tidb-4000/conf/tidb.toml
invalid config TiDB run with skip-grant-table need root privilege
, ssh_stdout: , ssh_command: export LANG=C; PATH=$PATH:/bin:/sbin:/usr/bin:/usr/sbin /tidb-deploy/tidb-4000/bin/tidb-server --config-check --config=/tidb-deploy/tidb-4000/conf/tidb.toml }, cause: Process exited with status 1: check config failed
由于权限的问题，并不能是参数生效，不过tidb-server节点还是可用状态
[root@node1 ~]# tiup cluster display tidb-test
tiup is checking updates for component cluster ...
Starting component `cluster`: /root/.tiup/components/cluster/v1.9.1/tiup-cluster /root/.tiup/components/cluster/v1.9.1/tiup-cluster display tidb-test
Cluster type: tidb
Cluster name: tidb-test
Cluster version: v5.0.0
Deploy user: tidb
SSH type: builtin
Dashboard URL: http://192.168.233.83:2379/dashboard
ID Role Host Ports OS/Arch Status Data Dir Deploy Dir
-- ---- ---- ----- ------- ------ -------- ----------
192.168.233.81:9093 alertmanager 192.168.233.81 9093/9094 linux/x86_64 Up /tidb-data/alertmanager-9093 /tidb-deploy/alertmanager-9093
192.168.233.81:3000 grafana 192.168.233.81 3000 linux/x86_64 Up - /tidb-deploy/grafana-3000
192.168.233.81:2379 pd 192.168.233.81 2379/2380 linux/x86_64 Up /tidb-data/pd-2379 /tidb-deploy/pd-2379
192.168.233.82:2379 pd 192.168.233.82 2379/2380 linux/x86_64 Up|L /tidb-data/pd-2379 /tidb-deploy/pd-2379
192.168.233.83:2379 pd 192.168.233.83 2379/2380 linux/x86_64 Up|UI /tidb-data/pd-2379 /tidb-deploy/pd-2379
192.168.233.81:9090 prometheus 192.168.233.81 9090 linux/x86_64 Up /tidb-data/prometheus-9090 /tidb-deploy/prometheus-9090
192.168.233.82:4000 tidb 192.168.233.82 4000/10080 linux/x86_64 Up - /tidb-deploy/tidb-4000
192.168.233.83:4000 tidb 192.168.233.83 4000/10080 linux/x86_64 Up - /tidb-deploy/tidb-4000
192.168.233.81:20160 tikv 192.168.233.81 20160/20180 linux/x86_64 Up /tidb-data/tikv-20160 /tidb-deploy/tikv-20160
192.168.233.82:20160 tikv 192.168.233.82 20160/20180 linux/x86_64 Up /tidb-data/tikv-20160 /tidb-deploy/tikv-20160
192.168.233.83:20160 tikv 192.168.233.83 20160/20180 linux/x86_64 Up /tidb-data/tikv-20160 /tidb-deploy/tikv-20160
还是需要跟方法一样，将一个tidb-server节点关闭以后，手动使用root用户执行启动脚本。