Update MultiGet to provide consistent CF view for kPersistedTier #13433
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@cbi42 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator. |
jaykorean
approved these changes
Mar 3, 2025
| @@ -2753,29 +2753,30 @@ Status DBImpl::MultiCFSnapshot(const ReadOptions& read_options, | |||
| // sure. | |||
| constexpr int num_retries = 3; | |||
| for (int i = 0; i < num_retries; ++i) { | |||
| last_try = (i == num_retries - 1); | |||
| acquire_mutex = ((i == num_retries - 1) && !read_options.snapshot) || | |||
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nit: I'd change the order.
acquire_mutex = read_options.read_tier == kPersistedTier || ((i == num_retries - 1) && !read_options.snapshot);
Also consider updating the comment above that if read_tier is kPersistentTier we won't bother do the two first try without lock and just acquire the mutex and get the SV.
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I added some comment. read_options.read_tier == kPersistedTier is usually false so I ordered it last.
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@cbi42 has updated the pull request. You must reimport the pull request before landing. |
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@cbi42 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator. |
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Summary: when reading with ReadOptions::read_tier = kPersistedTier and with a snapshot, MultiGet allows the case where some CF is read before a flush and some CF is read after the flush. This is not desirable, especially when atomic_flush is enabled and users use MultiGet to do some consistency checks on the data in SST files. This PR updates the code path for SuperVersion acquisition to get a consistent view across when kPersistedTier is used.
Test plan: a new unit test that could be flaky without this change.