LeiRui commented on issue #429: [IOTDB-205]Support storage-group-level data ttl URL: https://github.com/apache/incubator-iotdb/pull/429#issuecomment-542204822 Hi, This is a review on functionality. I find the operation `unset TTL to <storage group>` REALLY requires a seasoned user who knows exactly what he/she is doing. And I totally agree with what you have noted in the guidance: > Notice: When you set TTL to some storage groups, data out of the TTL will be made invisible immediately, but because the data files may contain both out-dated and living data or the data files may be being used by queries, the physical removal of data is stale. If you increase or unset TTL just after setting it previously, some previously invisible data may be seen again, but the physically removed one is lost forever. **In other word, different from delete statement, the atomicity of data deletion is not guaranteed for efficiency concerns.** **So we recommend that you do not change the TTL once it is set or at least do not reset it frequently, unless you are determined to suffer the unpredictability.** I want to draw attention to another point that can be easily misunderstood when using the operation `unset TTL to <storage group>`. That is, the latest insertion time of a storage group, which is the split point used to determine whether the coming data point is sequential or unsequential, will not be “recycled" even if the data before `now()-TTL` are really deleted.
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