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add more checks for background segment split #4406

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merged 3 commits into from
Mar 24, 2022
Merged

add more checks for background segment split #4406

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d48cebd
add more checks for background split
lidezhu Mar 24, 2022
b4bfff7
Merge branch 'master' into add-more-segment-split-check
lidezhu Mar 24, 2022
aa00479
Merge branch 'master' into add-more-segment-split-check
lidezhu Mar 24, 2022
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12 changes: 8 additions & 4 deletions dbms/src/Storages/DeltaMerge/DeltaMergeStore.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1196,9 +1196,13 @@ void DeltaMergeStore::checkSegmentUpdate(const DMContextPtr & dm_context, const

// Note that, we must use || to combine rows and bytes checks in split check, and use && in merge check.
// Otherwise, segments could be split and merged over and over again.
bool should_split = (segment_rows >= segment_limit_rows * 2 || segment_bytes >= segment_limit_bytes * 2)
&& (delta_rows - delta_last_try_split_rows >= delta_cache_limit_rows
|| delta_bytes - delta_last_try_split_bytes >= delta_cache_limit_bytes);
// Do background split in the following two cases:
// 1. The segment is large enough, and there are some data in the delta layer. (A hot segment which is large enough)
// 2. The segment is too large. (A segment which is too large, although it is cold)
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how do we trigger checkSegmentUpdate on a segment that is cold?

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Here is some intuition for the check condition in the comments. The actual problem is here: https://docs.google.com/document/d/1tkJXERVmcKv6nwfhTXqk0qTdVm2CPh8OXZiZ8ap6sOA/edit?usp=sharing.

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"Cold" means no continuous small write in this segment.

bool should_bg_split = ((segment_rows >= segment_limit_rows * 2 || segment_bytes >= segment_limit_bytes * 2)
&& (delta_rows - delta_last_try_split_rows >= delta_cache_limit_rows
|| delta_bytes - delta_last_try_split_bytes >= delta_cache_limit_bytes))
|| (segment_rows >= segment_limit_rows * 3 || segment_bytes >= segment_limit_bytes * 3);

bool should_merge = segment_rows < segment_limit_rows / 3 && segment_bytes < segment_limit_bytes / 3;

Expand Down Expand Up @@ -1310,7 +1314,7 @@ void DeltaMergeStore::checkSegmentUpdate(const DMContextPtr & dm_context, const
return false;
};
auto try_bg_split = [&](const SegmentPtr & seg) {
if (should_split && !seg->isSplitForbidden())
if (should_bg_split && !seg->isSplitForbidden())
{
delta_last_try_split_rows = delta_rows;
delta_last_try_split_bytes = delta_bytes;
Expand Down