After a TDengine cluster has been running for a long enough time, because of data insertion, table deletion and deletion of expired data, there may be fragments in data files and query performance may be impacted. To resolve the problem of fragments, since version 184.108.40.206 a new SQL command
COMPACT can be used to defragment data files.
COMPACT VNODES IN (vg_id1, vg_id2, ...)
COMPACT can be used to defragment one or more vgroups. The defragmentation work will be scheduled in the task queue for execution by TDengine.
SHOW VGROUPS command can be used to get the vgroup ids to be used in
COMPACT command. There is a column
compacting in the output of
SHOW GROUPS to indicate the compaction status of the vgroup: 2 means the vgroup is waiting in task queue for compaction, 1 means compaction is in progress, and 0 means the vgroup has not been scheduled for compaction.
Please note that a lot of disk I/O is required for defragementation operations. During defragmentation the performance may be impacted significantly for data insertion and query. Data insertion may even be blocked for very short periods, in extreme cases.
Optimize Storage Parameters
The data in different use cases may have different characteristics, such as the days to keep, number of replicas, collection interval, record size, number of collection points, compression or not, etc. To achieve best efficiency in storage, the parameters in the table below can be used. All of them can either be configured in
taos.cfg, as default parameters, or can be set in the command
create database. For detailed definition of these parameters please refer to Configuration Parameters.
|The time range of the data stored in a single data file
|The number of days the data is kept in the database
|The size of each memory block
|The number of memory blocks used by each vnode
|The number of required confirmations in case of multiple replicas
|The minimum number of rows in a data file
|The maximum number of rows in a data file
|Whether to compress the data
|0：uncompressed; 1: One Phase compression; 2: Two Phase compression
|wal sync level (named as "wal" in create database )
|1：wal enabled without fsync; 2：wal enabled with fsync
|The time to wait for invoking fsync when walLevel is set to 2; 0 means no wait
|The number of replications
|ms: millisecond; us: microsecond;ns: nanosecond
|Whether to allow updating data
|0: not allowed; 1: a whole row must be updated; 2: a portion of columns in a row can be updated
|Whether the latest data of a table is cached in memory
|0: not cached; 1: the last row is cached; 2: the latest non-NULL value of each column is cached
Even for a specific use case, there may be multiple kinds of data with different characteristics. In this case it's best to put data with the same characteristics in the same database. There may be multiple databases in a system and each database can be configured with different storage parameters to achieve the best performance. The above parameters can be used when creating a database to override the default setting in the configuration file.
CREATE DATABASE demo DAYS 10 CACHE 32 BLOCKS 8 REPLICA 3 UPDATE 1;
The above SQL statement creates a database named
demo, in which each data file stores 10 days of data, the size of each memory block is 32 MB and 8 blocks are allocated to each vnode, there are 3 replicas and update operations are allowed. All other parameters not specified in the command, will default to the values in the configuration file
Once a database is created, only some parameters can be changed and be effective immediately while others are can't.
|ALTER DATABASE <dbname> REPLICA n
|ALTER DATABASE <dbname> QUORUM n
|ALTER DATABASE <dbname> KEEP n
|ALTER DATABASE <dbname> BLOCKS n
|ALTER DATABASE <dbname> COMP n
|0 | 1 | 2 | 3
|ALTER DATABASE <dbname> CACHELAST n
Explanation： Prior to version 220.127.116.11,
taosd server process needs to be restarted for these parameters to take in effect if they are changed using
When trying to join a new dnode into a running TDengine cluster, all the parameters related to the cluster in the new dnode configuration must be consistent with the cluster, otherwise it can't join the cluster. The parameters that are checked when joining a dnode are listed below. For detailed definition of these parameters please refer to Configuration Parameters.
For the convenience of debugging, the log setting of a dnode can be changed temporarily. The temporary change will be lost once the server is restarted.
ALTER DNODE <dnode_id> <config>
- dnode_id: from output of "SHOW DNODES"
- config: the parameter to be changed, as below
- resetlog: close the old log file and create the new one
- debugFlag: 131 (INFO/ERROR/WARNING), 135 (DEBUG), 143 (TRACE)
alter dnode 1 debugFlag 135;