diff --git a/docs/en/07-develop/08-cache.md b/docs/en/07-develop/08-cache.md index 6a6ca3e5947b6a3233f90e5e0a01417e6be3b19d..9ef5d2c4f0c7697a1bdbb45c84ff28d3531417d8 100644 --- a/docs/en/07-develop/08-cache.md +++ b/docs/en/07-develop/08-cache.md @@ -10,10 +10,10 @@ TDengine uses various kinds of caching techniques to efficiently write and query TDengine uses an insert-driven cache management policy, known as first in, first out (FIFO). This policy differs from read-driven "least recently used (LRU)" cache management. A FIFO policy stores the latest data in cache and flushes the oldest data from cache to disk when the cache usage reaches a threshold. In IoT use cases, the most recent data or the current state is most important. The cache policy in TDengine, like much of the design and architecture of TDengine, is based on the nature of IoT data. -When you create a database, you can configure the size of the write cache on each vnode. The **vgroups** parameter determines the number of vgroups that process data in the database, and the **buffer** parameter determines the size of the write cache for each vnode. +When you create a database, you can configure the size of the write cache on each vnode. The **vgroups** parameter determines the number of vgroups that process data in the database, and the **buffer** parameter determines the size of the write cache for each vnode. The unit of buffer is MB. ```sql -create database db0 vgroups 100 buffer 16MB +create database db0 vgroups 100 buffer 16 ``` In theory, larger cache sizes are always better. However, at a certain point, it becomes impossible to improve performance by increasing cache size. In most scenarios, you can retain the default cache settings. @@ -28,10 +28,10 @@ When you create a database, you can configure whether the latest data from every ## Metadata Cache -To improve query and write performance, each vnode caches the metadata that it receives. When you create a database, you can configure the size of the metadata cache through the *pages* and *pagesize* parameters. +To improve query and write performance, each vnode caches the metadata that it receives. When you create a database, you can configure the size of the metadata cache through the *pages* and *pagesize* parameters. The unit of pagesize is kb. ```sql -create database db0 pages 128 pagesize 16kb +create database db0 pages 128 pagesize 16 ``` The preceding SQL statement creates 128 pages on each vnode in the `db0` database. Each page has a 16 KB metadata cache. diff --git a/docs/zh/07-develop/08-cache.md b/docs/zh/07-develop/08-cache.md index 29e28e3dde0816d9e5a08f74abd2382854d336da..07479b9a27e12745101898e9a8e046b5f169adc1 100644 --- a/docs/zh/07-develop/08-cache.md +++ b/docs/zh/07-develop/08-cache.md @@ -10,10 +10,10 @@ description: "TDengine 内部的缓存设计" TDengine 采用时间驱动缓存管理策略（First-In-First-Out，FIFO），又称为写驱动的缓存管理机制。这种策略有别于读驱动的数据缓存模式（Least-Recent-Used，LRU），直接将最近写入的数据保存在系统的缓存中。当缓存达到临界值的时候，将最早的数据批量写入磁盘。一般意义上来说，对于物联网数据的使用，用户最为关心最近产生的数据，即当前状态。TDengine 充分利用了这一特性，将最近到达的（当前状态）数据保存在缓存中。 -每个 vnode 的写入缓存大小在创建数据库时决定，创建数据库时的两个关键参数 vgroups 和 buffer 分别决定了该数据库中的数据由多少个 vgroup 处理，以及向其中的每个 vnode 分配多少写入缓存。 +每个 vnode 的写入缓存大小在创建数据库时决定，创建数据库时的两个关键参数 vgroups 和 buffer 分别决定了该数据库中的数据由多少个 vgroup 处理，以及向其中的每个 vnode 分配多少写入缓存。buffer 的单位是MB。 ```sql -create database db0 vgroups 100 buffer 16MB +create database db0 vgroups 100 buffer 16 ``` 理论上缓存越大越好，但超过一定阈值后再增加缓存对写入性能提升并无帮助，一般情况下使用默认值即可。 @@ -28,10 +28,10 @@ create database db0 vgroups 100 buffer 16MB ## 元数据缓存 -为了更高效地处理查询和写入，每个 vnode 都会缓存自己曾经获取到的元数据。元数据缓存由创建数据库时的两个参数 pages 和 pagesize 决定。 +为了更高效地处理查询和写入，每个 vnode 都会缓存自己曾经获取到的元数据。元数据缓存由创建数据库时的两个参数 pages 和 pagesize 决定。pagesize 的单位是 kb。 ```sql -create database db0 pages 128 pagesize 16kb +create database db0 pages 128 pagesize 16 ``` 上述语句会为数据库 db0 的每个 vnode 创建 128 个 page，每个 page 16kb 的元数据缓存。