store: Add status variables for tikv statistics

[morgo]

What problem does this PR solve?

Problem Summary:

In the DBaaS, the table mysql.tidb will be hidden (enforced by Security Enhanced mode, WIP).

This follows on from #21988 and mirrors the tikv statistics in SHOW GLOBAL STATUS. For non-DBaaS installs, the reading from mysql.tidb will continue to be supported (although for consistency between the two, documentation will be updated to point to SHOW GLOBAL STATUS).

What is changed and how it works?

How it Works:

TiDB does not make much use of STATUS variables, but it is the standard way for MySQL to expose various statistics. Here is an example with this added:

mysql> SHOW GLOBAL STATUS;
+-----------------------+------------------------------------------------------------------------------------------+
| Variable_name         | Value                                                                                    |
+-----------------------+------------------------------------------------------------------------------------------+
| Ssl_cipher            |                                                                                          |
| Ssl_cipher_list       |                                                                                          |
| Ssl_verify_mode       | 0                                                                                        |
| Ssl_version           |                                                                                          |
| ddl_schema_version    | 24                                                                                       |
| server_id             | 0d2ce36b-8c20-4610-b95e-488488e1da17                                                     |
| tidb_gc_last_run_time | 20210107-17:13:14 -0700                                                                  |
| tidb_gc_leader_desc   | host:ubuntu, pid:731438, start at 2021-01-07 16:13:14.776532333 -0700 MST m=+4.599786612 |
| tidb_gc_leader_lease  | 20210107-17:15:14 -0700                                                                  |
| tidb_gc_leader_uuid   | 5db7c8098480009                                                                          |
| tidb_gc_safe_point    | 20210107-17:03:14 -0700                                                                  |
+-----------------------+------------------------------------------------------------------------------------------+
11 rows in set (0.01 sec)

mysql> SHOW GLOBAL STATUS LIKE 'tidb_gc_safe_point';
+--------------------+-------------------------+
| Variable_name      | Value                   |
+--------------------+-------------------------+
| tidb_gc_safe_point | 20210107-17:03:14 -0700 |
+--------------------+-------------------------+
1 row in set (0.01 sec)

Related changes

• Docs will need to be updated

Check List

Tests

• Unit test

Side effects

• None (change is backwards compatible)
• Increased complexity/usability (there is two ways of doing something..)

Release note

• Garbage collection statistics are now available via the command SHOW GLOBAL STATUS.

[youjiali1995]

LGTM

[MyonKeminta]

Rest LGTM

[MyonKeminta]

The leader desc usually contains the leader node's hostname or IP address. Since mysql.tidb will be hidden for security concern, can this be exposed directly?

[morgo]

We will have to add filtering of SHOW [GLOBAL] STATUS to security enhanced mode. The proposal is closed for now because it was moved out of this sprint. cc @SunRunAway

Moving to SHOW STATUS is still beneficial though, since filtering from here has a cleaner semantic than patching out results from SELECT * FROM mysql.tidb.

[MyonKeminta]

LGTM.

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[morgo]

/merge

[ti-chi-bot]

This pull request has been accepted and is ready to merge.

Commit hash: 42e101c

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[Rustin170506]

/run-unit-test