flush cache before segment merge #4955

lidezhu · 2022-05-22T09:21:39Z

What problem does this PR solve?

Issue Number: close #4956

Problem Summary:
When do segment split, we try to copy the tail column files in the delta layer of the original segment to the new result
segments. So the new segments may contain data that doesn't belong to its segment range.

And this is ok for most cases, because the redundant data will be filtered out by the segment range when serve the read requests to the segment. So the redundant is invisible in almost all cases.

But when do segment merge later, if the previous redundant data is still not flushed to disk, it will be directly copied to the new merged segment again.

So the redundant data in each segment become visible again after segment merge which may cause potential data incorrectness.

What is changed and how it works?

Flush cache before every merge operation. So the potential unsaved data will be filtered out by the segment range when do merge.

Check List

Tests

Unit test
Integration test
Manual test (add detailed scripts or steps below)
No code

Side effects

Performance regression: Consumes more CPU
Performance regression: Consumes more Memory
Breaking backward compatibility

Documentation

Release note

Fix potential wrong result after a lot of insert and delete operations

ti-chi-bot · 2022-05-22T09:21:40Z

[REVIEW NOTIFICATION]

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JaySon-Huang
flowbehappy

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lidezhu · 2022-05-22T09:37:48Z

/run-all-tests

dbms/src/Storages/DeltaMerge/DeltaMergeStore.cpp

dbms/src/Storages/DeltaMerge/Segment.cpp

sre-bot · 2022-05-22T10:08:34Z

Coverage for changed files

Filename                       Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DeltaMergeStore.cpp               1422               475    66.60%          67                 5    92.54%        2020               441    78.17%         822               370    54.99%
Segment.cpp                        650               185    71.54%          47                 3    93.62%        1144               248    78.32%         342               175    48.83%
Segment.h                           34                 5    85.29%          22                 3    86.36%          31                 4    87.10%           6                 3    50.00%
tests/gtest_dm_segment.cpp        3404               814    76.09%          38                 1    97.37%        1505                21    98.60%        1130               471    58.32%
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             5510              1479    73.16%         174                12    93.10%        4700               714    84.81%        2300              1019    55.70%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
18224      9785             46.31%    204149  97870        52.06%

full coverage report (for internal network access only)

lidezhu · 2022-05-22T10:38:56Z

/run-all-tests

sre-bot · 2022-05-22T10:55:30Z

Coverage for changed files

Filename                       Regions    Missed Regions     Cover   Functions  Missed Functions  Executed       Lines      Missed Lines     Cover    Branches   Missed Branches     Cover
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
DeltaMergeStore.cpp               1426               477    66.55%          67                 5    92.54%        2024               445    78.01%         826               373    54.84%
Segment.cpp                        654               187    71.41%          47                 3    93.62%        1148               252    78.05%         346               180    47.98%
Segment.h                           34                 5    85.29%          22                 3    86.36%          31                 4    87.10%           6                 3    50.00%
tests/gtest_dm_segment.cpp        3404               814    76.09%          38                 1    97.37%        1505                21    98.60%        1130               471    58.32%
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
TOTAL                             5518              1483    73.12%         174                12    93.10%        4708               722    84.66%        2308              1027    55.50%

Coverage summary

Functions  MissedFunctions  Executed  Lines   MissedLines  Cover
18224      9785             46.31%    204157  97924        52.03%

full coverage report (for internal network access only)

dbms/src/Storages/DeltaMerge/DeltaMergeStore.cpp

dbms/src/Storages/DeltaMerge/tests/gtest_dm_segment.cpp