GC Configuration
The GC (Garbage Collection) configuration and operational status are recorded in the mysql.tidb
system table. You can use SQL statements to query or modify them:
mysql> select VARIABLE_NAME, VARIABLE_VALUE from mysql.tidb;
+--------------------------+----------------------------------------------------------------------------------------------------+
| VARIABLE_NAME | VARIABLE_VALUE |
+--------------------------+----------------------------------------------------------------------------------------------------+
| bootstrapped | True |
| tidb_server_version | 33 |
| system_tz | UTC |
| tikv_gc_leader_uuid | 5afd54a0ea40005 |
| tikv_gc_leader_desc | host:tidb-cluster-tidb-0, pid:215, start at 2019-07-15 11:09:14.029668932 +0000 UTC m=+0.463731223 |
| tikv_gc_leader_lease | 20190715-12:12:14 +0000 |
| tikv_gc_enable | true |
| tikv_gc_run_interval | 10m0s |
| tikv_gc_life_time | 10m0s |
| tikv_gc_last_run_time | 20190715-12:09:14 +0000 |
| tikv_gc_safe_point | 20190715-11:59:14 +0000 |
| tikv_gc_auto_concurrency | true |
| tikv_gc_mode | distributed |
+--------------------------+----------------------------------------------------------------------------------------------------+
13 rows in set (0.00 sec)
For example, the following statement makes GC keep history data for the most recent 24 hours:
update mysql.tidb set VARIABLE_VALUE="24h" where VARIABLE_NAME="tikv_gc_life_time";
tikv_gc_enable
- Enables or disables GC
- Default:
true
tikv_gc_run_interval
- Specifies the GC interval, in the format of Go Duration, for example,
"1h30m"
, and"15m"
- Default:
"10m0s"
tikv_gc_life_time
The time limit during which data is retained for each GC, in the format of Go Duration. When a GC happens, the current time minus this value is the safe point.
Default:
"10m0s"
tikv_gc_mode
Specifies the GC mode. Possible values are:
"distributed"
(default): Distributed GC mode. In the Do GC step, the GC leader on the TiDB side uploads the safe point to PD. Each TiKV node obtains the safe point respectively and performs GC on all leader Regions on the current node. This mode is is supported from TiDB 3.0."central"
: Central GC mode. In the Do GC step, the GC leader sends GC requests to all Regions. This mode is adopted by TiDB 2.1 or earlier versions.
tikv_gc_auto_concurrency
Controls whether to let TiDB automatically specify the GC concurrency, or the maximum number of GC threads allowed concurrently.
When
tikv_gc_mode
is set to"distributed"
, GC concurrency works in the Resolve Locks step. Whentikv_gc_mode
is set to"central"
, it is applied to both the Resolve Locks and Do GC steps.true
(default): Automatically use the number of TiKV nodes in the cluster as the GC concurrencyfalse
: Use the value oftikv_gc_concurrency
as the GC concurrency
tikv_gc_concurrency
- Specifies the GC concurrency manually. This parameter works only when you set
tikv_gc_auto_concurrency
tofalse
. - Default: 2
Notes on GC process changes
Since TiDB 3.0, some configuration options have changed with support for the distributed GC mode and concurrent Resolve Locks processing. The changes are shown in the following table:
Version/Configuration | Resolve Locks | Do GC |
---|---|---|
2.x | Serial | Concurrent |
3.0 tikv_gc_mode = centered tikv_gc_auto_concurrency = false | Concurrent | Concurrent |
3.0 tikv_gc_mode = centered tikv_gc_auto_concurrency = true | Auto-concurrent | Auto-concurrent |
3.0 tikv_gc_mode = distributed tikv_gc_auto_concurrency = false | Concurrent | Distributed |
3.0 tikv_gc_mode = distributed tikv_gc_auto_concurrency = true (default) | Auto-concurrent | Distributed |
- Serial: requests are sent from TiDB Region by Region.
- Concurrent: requests are sent to each Region concurrently based on the number of threads specified in the
tikv_gc_concurrency
. - Auto-concurrent: requests are sent to each Region concurrently with the number of TiKV nodes as concurrency value.
- Distributed: no need for TiDB to send requests to TiKV to trigger GC because each TiKV handles GC on its own.
GC I/O limit
TiKV supports the GC I/O limit. You can configure gc.max-write-bytes-per-sec
to limit writes of a GC worker per second, and thus to reduce the impact on normal requests.
0
indicates disabling this feature.
You can dynamically modify this configuration using tikv-ctl:
tikv-ctl --host=ip:port modify-tikv-config -m server -n gc.max_write_bytes_per_sec -v 10MB