Introduce SuperSnapshot #292
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Just realized I also need to add support for MultiGet, adding that now. |
igorcanadi
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Added MultiGet support together with the unit test. |
| read_options.background_purge_on_iterator_cleanup || | ||
| immutable_db_options_.avoid_unnecessary_blocking_io); | ||
| internal_iter->RegisterCleanup(CleanupSuperVersionHandle, cleanup, nullptr); | ||
| // Do not clean up the super version if super snapshot owns it |
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Are we assuming that the super_version passed to NewInternalIterator is always the same as read_options.snapshot.sv_ here?
If so, how is that guarantee maintained?
If not, why can't we cleanup super_version here?
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Asking because NewInternalIterator is called in multiple places, but seems we only maintain the assumption in DBImpl::NewIteratorImpl.
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Yes, this is the assumption here, and you're right that it's not respected in some scenarios (like iterator refresh, although iterator refresh is also not supported if snapshot is non-nullptr). I wll add a check to only avoid cleaning up if super_version is not the same as read_options.snapshot.sv_.
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Addressed this comment in latest version of the code.
Summary: SuperSnapshot is a combination of a snapshot and a super-version. They fulfill the same goal as snapshots, i.e. provide a consistent view of the database. However, unlike snapshots, they also pin the current super-version (memtable, immutable memtable list and file LSM tree). In that way they are similar to iterators, which also pin the current super-version for the duration of their lifetime. Test Plan: Added a basic unit test. Reviewers:
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Summary:
SuperSnapshot is a combination of a snapshot and a super-version. They
fulfill the same goal as snapshots, i.e. provide a consistent view of
the database. However, unlike snapshots, they also pin the current
super-version (memtable, immutable memtable list and file LSM tree). In
that way they are similar to iterators, which also pin the current
super-version for the duration of their lifetime.
Test Plan:
Added a basic unit test.
Reviewers: