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 ` 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 `. 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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