SpaceTL Delete

1. Overview

IoTDB supports configuring the SpaceTL deletion policy through parameters: when the storage space of the last-level storage tier (object storage space is considered unlimited) is insufficient, users can choose whether to trigger the SpaceTL deletion mechanism to free up space.

Note: This mechanism only deletes files in the last storage tier.

2. Parameter Introduction

Parameter Name	Description	Default Value	Effective Mode
dn_tier_full_policy	Behavior policy when the storage space of the last-level storage tier is insufficient. Configurable options:1. NULL: Take no action.2. DELETE: Enable SpaceTL data deletion.	NULL	Hot Reload

Parameter Explanation:

• If the last-level storage tier includes object storage, SpaceTL will not be triggered.
• This parameter works in conjunction with the dn_default_space_usage_thresholdsparameter. SpaceTL is triggered when the available space of the last-level storage tier falls below the space_usage_thresholds. By default, deletion begins when disk space usage reaches 85% and continues until usage drops below 85%.

3. Usage Example

In a three-tier storage setup, when the storage space of the last tier reaches 85%, the SpaceTL deletion mechanism is triggered to start deleting files.

# 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

In this example, three tiers of storage are configured as follows:

Tier Level	Storage Directories	Data Retention Time	Disk Storage Space Threshold
Tier 1	/data1/data	Retained for 1 day; exceeded data moves to Tier 2	Data moves to Tier 2 when disk usage reaches 75%
Tier 2	/data2/data,/data3/data	Retained for 10 days; exceeded data moves to Tier 3	Data moves to Tier 3 when disk usage reaches 80%
Tier 3	/data4/data	No expiration time; data is stored indefinitely	Deletes some files when disk usage reaches 85% (due todn_tier_full_policy=DELETE)