src/zh/UserGuide/Master/Table/Basic-Concept/SpaceTL-Delete.md

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# SpaceTL 删除

## 1. 概述

IoTDB 支持通过参数配置 SpaceTL 删除策略：即当最后一级存储层（对象存储空间被视为无限）的存储空间不足时，可选择是否触发 SpaceTL 删除机制以释放空间。

注意：该机制仅会删除最后一个存储层中的文件。

## 2. 参数介绍

| 参数名                 | 描述                                                                                                    | 默认值 | 生效方式 |
| ------------------------ | --------------------------------------------------------------------------------------------------------- | -------- | ---------- |
| dn\_tier\_full\_policy | 最后一级存储层存储空间不足时的行为策略。可配置：1. NULL：不做任何处理1. DELETE：开启 SpaceTL 删除数据。 | NULL   | 热加载   |

参数说明：

* 若最后一级存储层包含**对象存储**则不触发 SpaceTL
* 该参数和 dn\_default\_space\_usage\_thresholds 参数联动，当最后一级存储层的可用空间低于 space\_usage\_thresholds 时触发 SpaceTL；默认当磁盘空间占用到达 85% 的时候开始删除直至空间占用降到 85% 以下才会停止

## 3. 使用示例

以三级存储为例，当最后一级存储空间到达85%时，触发 SpaceTL 删除机制开始删除文件。

```Properties
# data dirs
# If this property is unset, system will save the data in the default relative path directory under the IoTDB folder(i.e., %IOTDB_HOME%/data/datanode/data).
# If it is absolute, system will save the data in exact location it points to.
# If it is relative, system will save the data in the relative path directory it indicates under the IoTDB folder.
# If there are more than one directory, please separate them by commas ",".
# If tiered storage is enabled, please separate directories of different tiers by semicolons ";".
# Use keyword OBJECT_STORAGE to denote object storage media, object storage can only exist on the last tier and must have local dirs in the front tiers.
# Note: If data_dirs is assigned an empty string(i.e.,zero-size), it will be handled as a relative path.
# effectiveMode: hot_reload
# For windows platform
# If its prefix is a drive specifier followed by "\\", or if its prefix is "\\\\", then the path is absolute. Otherwise, it is relative.
# dn_data_dirs=data\\datanode\\data
# For Linux platform
# If its prefix is "/", then the path is absolute. Otherwise, it is relative.
dn_data_dirs=/data1/data;/data2/data,/data3/data;/data4/data

# Default tier TTL. When the survival time of the data exceeds the threshold, it will be migrated to the next tier.
# Negative value means the tier TTL is unlimited.
# effectiveMode: restart
# Datatype: long
# Unit: ms
tier_ttl_in_ms=86400000;864000000;-1

# Disk usage threshold, data will be moved to the next tier when the usage of the tier is higher than this threshold.
# If tiered storage is enabled, please separate thresholds of different tiers by semicolons ";".
# effectiveMode: hot_reload
# Datatype: double(percentage)
dn_default_space_usage_thresholds=0.75;0.8;0.85

# How to deal with the last tier?s data when its used space has been higher than its dn_default_space_usage_thresholds.
# The info of the two policies are as follows:
# effectiveMode: hot_reload
# 1. NULL: do nothing.
# 2. DELETE: delete the oldest data.
dn_tier_full_policy=DELETE
```

在该示例中，共配置了三个层级的存储，具体为：

| 层级   | 存储目录                | 数据保留时间                      | 磁盘存储空间阈值                                                 |
| -------- | ------------------------- | ----------------------------------- | ------------------------------------------------------------------ |
| 层级一 | /data1/data             | 只保留 1 天，超过后会移动到第二级 | 磁盘达到 75% 会移动部分文件到第二级                              |
| 层级二 | /data2/data,/data3/data | 只保留 10 天，超过后到第三级      | 磁盘达到 80% 会移动部分文件到第三级                              |
| 层级三 | /data4/data             | 没有过期时间，会一直存储          | 磁盘达到 85% 会删除部分文件（因为dn\_tier\_full\_policy=DELETE） |