Titan Configuration

This document introduces how to enable and disable Titan using the corresponding configuration items, as well as the relevant parameters and the Level Merge feature.

Enable Titan

Titan is compatible with RocksDB, so you can directly enable Titan on the existing TiKV instances that use RocksDB. You can use one of the following two methods to enable Titan:

• Method 1: If you have deployed the cluster using TiUP, you can execute the tiup cluster edit-config ${cluster-name} command and edit the TiKV configuration file as the following example shows:

    tikv:
      rocksdb.titan.enabled: true
  

  Reload the configuration and TiKV will be rolling restarted online:

  tiup cluster reload ${cluster-name} -R tikv
  

  For the detailed command, see Modify the configuration using TiUP.

• Method 2: Directly edit the TiKV configuration file to enable Titan (NOT recommended for the production environment).

  [rocksdb.titan]
  enabled = true
  

After Titan is enabled, the existing data stored in RocksDB is not immediately moved to the Titan engine. As new data is written to the TiKV foreground and RocksDB performs compaction, the values are progressively separated from keys and written to Titan. You can view the TiKV Details -> Titan kv -> blob file size panel to confirm the size of the data stored in Titan.

If you want to speed up the writing process, compact data of the whole TiKV cluster manually using tikv-ctl. For details, see manual compaction.

Parameters

To adjust Titan-related parameters using TiUP, refer to Modify the configuration.

• Titan GC thread count.

  From the TiKV Details -> Thread CPU -> RocksDB CPU panel, if you observe that the Titan GC threads are at full capacity for a long time, consider increasing the size of the Titan GC thread pool.

  [rocksdb.titan]
  max-background-gc = 1
  

• Value size threshold.

  When the size of the value written to the foreground is smaller than the threshold, this value is stored in RocksDB; otherwise, this value is stored in the blob file of Titan. Based on the distribution of value sizes, if you increase the threshold, more values are stored in RocksDB and TiKV performs better in reading small values. If you decrease the threshold, more values go to Titan, which further reduces RocksDB compactions.

  [rocksdb.defaultcf.titan]
  min-blob-size = "1KB"
  

• The algorithm used for compressing values in Titan, which takes value as the unit.

  [rocksdb.defaultcf.titan]
  blob-file-compression = "lz4"
  

• The size of value caches in Titan.

  Larger cache size means higher read performance of Titan. However, too large a cache size causes Out of Memory (OOM). It is recommended to set the value of storage.block-cache.capacity to the store size minus the blob file size and set blob-cache-size to memory size * 50% - block cache size according to the monitoring metrics when the database is running stably. This maximizes the blob cache size when the block cache is large enough for the whole RocksDB engine.

  [rocksdb.defaultcf.titan]
  blob-cache-size = 0
  

• When the ratio of discardable data (the corresponding key has been updated or deleted) in a blob file exceeds the following threshold, Titan GC is triggered.

  discardable-ratio = 0.5
  

  When Titan writes the useful data of this blob file to another file, you can use the discardable-ratio value to estimate the upper limits of write amplification and space amplification (assuming the compression is disabled).

  Upper limit of write amplification = 1 / discardable_ratio

  Upper limit of space amplification = 1 / (1 - discardable_ratio)

  From the two equations above, you can see that decreasing the value of discardable_ratio can reduce space amplification but causes GC to be more frequent in Titan. Increasing the value reduces Titan GC, the corresponding I/O bandwidth, and CPU consumption but increases disk usage.

• The following option limits the I/O rate of RocksDB compaction. During peak traffic, limiting RocksDB compaction, its I/O bandwidth, and its CPU consumption reduces its impact on the write and read performance of the foreground.

  When Titan is enabled, this option limits the summed I/O rates of RocksDB compaction and Titan GC. If you find that the I/O and/or CPU consumption of RocksDB compaction and Titan GC is too large, set this option to a suitable value according the disk I/O bandwidth and the actual write traffic.

  [rocksdb]
  rate-bytes-per-sec = 0
  

Disable Titan (experimental)

To disable Titan, you can configure the rocksdb.defaultcf.titan.blob-run-mode option. The optional values for blob-run-mode are as follows:

• When the option is set to normal, Titan performs read and write operations normally.
• When the option is set to read-only, all newly written values are written into RocksDB, regardless of the value size.
• When the option is set to fallback, all newly written values are written into RocksDB, regardless of the value size. Also, all compacted values stored in the Titan blob file are automatically moved back to RocksDB.

To disable Titan, set blob-run-mode = "fallback" and perform a full compaction using tikv-ctl. After that, check the monitoring metrics, confirm that the blob file size decreases to 0. Then you can set rocksdb.titan.enabled to false and restart TiKV.

Level Merge (experimental)

In TiKV 4.0, Level Merge, a new algorithm, is introduced to improve the performance of range query and to reduce the impact of Titan GC on the foreground write operations. You can enable Level Merge using the following option:

[rocksdb.defaultcf.titan]
level-merge = true

Enabling Level Merge has the following benefits:

• Greatly improve the performance of Titan range query.
• Reduce the impact of Titan GC on the foreground write operations and improve write performance.
• Reduce space amplification of Titan and the disk usage (compared to the disk usage with the default configuration).

Accordingly, the write amplification with Level Merge enabled is slightly higher than that of Titan but is still lower than that of the native RocksDB.