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Peter Dillinger
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cb08a682d4 |
Fix/cleanup SeqnoToTimeMapping (#12253)
Summary: The SeqnoToTimeMapping class (RocksDB internal) used by the preserve_internal_time_seconds / preclude_last_level_data_seconds options was essentially in a prototype state with some significant flaws that would risk biting us some day. This is a big, complicated change because both the implementation and the behavioral requirements of the class needed to be upgraded together. In short, this makes SeqnoToTimeMapping more internally responsible for maintaining good invariants, so that callers don't easily encounter dangerous scenarios. * Some API functions were confusingly named and structured, so I fully refactored the APIs to use clear naming (e.g. `DecodeFrom` and `CopyFromSeqnoRange`), object states, function preconditions, etc. * Previously the object could informally be sorted / compacted or not, and there was limited checking or enforcement on these states. Now there's a well-defined "enforced" state that is consistently checked in debug mode for applicable operations. (I attempted to create a separate "builder" class for unenforced states, but IIRC found that more cumbersome for existing uses than it was worth.) * Previously operations would coalesce data in a way that was better for `GetProximalTimeBeforeSeqno` than for `GetProximalSeqnoBeforeTime` which is odd because the latter is the only one used by DB code currently (what is the seqno cut-off for data definitely older than this given time?). This is now reversed to consistently favor `GetProximalSeqnoBeforeTime`, with that logic concentrated in one place: `SeqnoToTimeMapping::SeqnoTimePair::Merge()`. Unfortunately, a lot of unit test logic was specifically testing the old, suboptimal behavior. * Previously, the natural behavior of SeqnoToTimeMapping was to THROW AWAY data needed to get reasonable answers to the important `GetProximalSeqnoBeforeTime` queries. This is because SeqnoToTimeMapping only had a FIFO policy for staying within the entry capacity (except in aggregate+sort+serialize mode). If the DB wasn't extremely careful to avoid gathering too many time mappings, it could lose track of where the seqno cutoff was for cold data (`GetProximalSeqnoBeforeTime()` returning 0) and preventing all further data migration to the cold tier--until time passes etc. for mappings to catch up with FIFO purging of them. (The problem is not so acute because SST files contain relevant snapshots of the mappings, but the problem would apply to long-lived memtables.) * Now the SeqnoToTimeMapping class has fully-integrated smarts for keeping a sufficiently complete history, within capacity limits, to give good answers to `GetProximalSeqnoBeforeTime` queries. * Fixes old `// FIXME: be smarter about how we erase to avoid data falling off the front prematurely.` * Fix an apparent bug in how entries are selected for storing into SST files. Previously, it only selected entries within the seqno range of the file, but that would easily leave a gap at the beginning of the timeline for data in the file for the purposes of answering GetProximalXXX queries with reasonable accuracy. This could probably lead to the same problem discussed above in naively throwing away entries in FIFO order in the old SeqnoToTimeMapping. The updated testing of GetProximalSeqnoBeforeTime in BasicSeqnoToTimeMapping relies on the fixed behavior. * Fix a potential compaction CPU efficiency/scaling issue in which each compaction output file would iterate over and sort all seqno-to-time mappings from all compaction input files. Now we distill the input file entries to a constant size before processing each compaction output file. Intended follow-up (me or others): * Expand some direct testing of SeqnoToTimeMapping APIs. Here I've focused on updating existing tests to make sense. * There are likely more gaps in availability of needed SeqnoToTimeMapping data when the DB shuts down and is restarted, at least with WAL. * The data tracked in the DB could be kept more accurate and limited if it used the oldest seqno of unflushed data. This might require some more API refactoring. Pull Request resolved: https://github.com/facebook/rocksdb/pull/12253 Test Plan: unit tests updated Reviewed By: jowlyzhang Differential Revision: D52913733 Pulled By: pdillinger fbshipit-source-id: 020737fcbbe6212f6701191a6ab86565054c9593 |
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Peter Dillinger
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1d5bddbc58 |
Bootstrap, pre-populate seqno_to_time_mapping (#11922)
Summary: This change has two primary goals (follow-up to https://github.com/facebook/rocksdb/issues/11917, https://github.com/facebook/rocksdb/issues/11920): * Ensure the DB seqno_to_time_mapping has entries that allow us to put a good time lower bound on any writes that happen after setting up preserve/preclude options (either in a new DB, new CF, SetOptions, etc.) and haven't yet aged out of that time window. This allows us to remove a bunch of work-arounds in tests. * For new DBs using preserve/preclude options, automatically reserve some sequence numbers and pre-map them to cover the time span back to the preserve/preclude cut-off time. In the future, this will allow us to import data from another DB by key, value, and write time by assigning an appropriate seqno in this DB for that write time. Note that the pre-population (historical mappings) does not happen if the original options at DB Open time do not have preserve/preclude, so it is recommended to create initial column families at that time with create_missing_column_families, to take advantage of this (future) feature. (Adding these historical mappings after DB Open would risk non-monotonic seqno_to_time_mapping, which is dubious if not dangerous.) Recommended follow-up: * Solve existing race conditions (not memory safety) where parallel operations like CreateColumnFamily or SetDBOptions could leave the wrong setting in effect. * Make SeqnoToTimeMapping more gracefully handle a possible case in which too many mappings are added for the time range of concern. It seems like there could be cases where data is massively excluded from the cold tier because of entries falling off the front of the mapping list (causing GetProximalSeqnoBeforeTime() to return 0). (More investigation needed.) No release note for the minor bug fix because this is still an experimental feature with limited usage. Pull Request resolved: https://github.com/facebook/rocksdb/pull/11922 Test Plan: tests added / updated Reviewed By: jowlyzhang Differential Revision: D49956563 Pulled By: pdillinger fbshipit-source-id: 92beb918c3a298fae9ca8e509717b1067caa1519 |
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Peter Dillinger
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02443dd93f |
Refactor, clean up, fixes, and more testing for SeqnoToTimeMapping (#11905)
Summary: This change is before a planned DBImpl change to ensure all sufficiently recent sequence numbers since Open are covered by SeqnoToTimeMapping (bug fix with existing test work-arounds). **Intended follow-up** However, I found enough issues with SeqnoToTimeMapping to warrant this PR first, including very small fixes in DB implementation related to API contract of SeqnoToTimeMapping. Functional fixes / changes: * This fixes some mishandling of boundary cases. For example, if the user decides to stop writing to DB, the last written sequence number would perpetually have its write time updated to "now" and would always be ineligible for migration to cold tier. Part of the problem is that the SeqnoToTimeMapping would return a seqno known to have been written before (immediately or otherwise) the requested time, but compaction_job.cc would include that seqno in the preserve/exclude set. That is fixed (in part) by adding one in compaction_job.cc * That problem was worse because a whole range of seqnos could be updated perpetually with new times in SeqnoToTimeMapping::Append (if no writes to DB). That logic was apparently optimized for GetOldestApproximateTime (now GetProximalTimeBeforeSeqno), which is not used in production, to the detriment of GetOldestSequenceNum (now GetProximalSeqnoBeforeTime), which is used in production. (Perhaps plans changed during development?) This is fixed in Append to optimize for accuracy of GetProximalSeqnoBeforeTime. (Unit tests added and updated.) * Related: SeqnoToTimeMapping did not have a clear contract about the relationships between seqnos and times, just the idea of a rough correspondence. Now the class description makes it clear that the write time of each recorded seqno comes before or at the associated time, to support getting best results for GetProximalSeqnoBeforeTime. And this makes it easier to make clear the contract of each API function. * Update `DBImpl::RecordSeqnoToTimeMapping()` to follow this ordering in gathering samples. Some part of these changes has required an expanded test work-around for the problem (see intended follow-up above) that the DB does not immediately ensure recent seqnos are covered by its mapping. These work-arounds will be removed with that planned work. An apparent compaction bug is revealed in PrecludeLastLevelTest::RangeDelsCauseFileEndpointsToOverlap, so that test is disabled. Filed GitHub issue #11909 Cosmetic / code safety things (not exhaustive): * Fix some confusing names. * `seqno_time_mapping` was used inconsistently in places. Now just `seqno_to_time_mapping` to correspond to class name. * Rename confusing `GetOldestSequenceNum` -> `GetProximalSeqnoBeforeTime` and `GetOldestApproximateTime` -> `GetProximalTimeBeforeSeqno`. Part of the motivation is that our times and seqnos here have the same underlying type, so we want to be clear about which is expected where to avoid mixing. * Rename `kUnknownSeqnoTime` to `kUnknownTimeBeforeAll` because the value is a bad choice for unknown if we ever add ProximalAfterBlah functions. * Arithmetic on SeqnoTimePair doesn't make sense except for delta encoding, so use better names / APIs with that in mind. * (OMG) Don't allow direct comparison between SeqnoTimePair and SequenceNumber. (There is no checking that it isn't compared against time by accident.) * A field name essentially matching the containing class name is a confusing pattern (`seqno_time_mapping_`). * Wrap calls to confusing (but useful) upper_bound and lower_bound functions to have clearer names and more code reuse. Pull Request resolved: https://github.com/facebook/rocksdb/pull/11905 Test Plan: GetOldestSequenceNum (now GetProximalSeqnoBeforeTime) and TruncateOldEntries were lacking unit tests, despite both being used in production (experimental feature). Added those and expanded others. Reviewed By: jowlyzhang Differential Revision: D49755592 Pulled By: pdillinger fbshipit-source-id: f72a3baac74d24b963c77e538bba89a7fc8dce51 |
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Jay Zhuang
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c401f285c3 |
Add option preserve_internal_time_seconds to preserve the time info (#10747)
Summary: Add option `preserve_internal_time_seconds` to preserve the internal time information. It's mostly for the migration of the existing data to tiered storage ( `preclude_last_level_data_seconds`). When the tiering feature is just enabled, the existing data won't have the time information to decide if it's hot or cold. Enabling this feature will start collect and preserve the time information for the new data. Pull Request resolved: https://github.com/facebook/rocksdb/pull/10747 Reviewed By: siying Differential Revision: D39910141 Pulled By: siying fbshipit-source-id: 25c21638e37b1a7c44006f636b7d714fe7242138 |
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Jay Zhuang
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faa0f9723c |
Tiered compaction: integrate Seqno time mapping with per key placement (#10370)
Summary: Using the Sequence number to time mapping to decide if a key is hot or not in compaction and place it in the corresponding level. Note: the feature is not complete, level compaction will run indefinitely until all penultimate level data is cold and small enough to not trigger compaction. Pull Request resolved: https://github.com/facebook/rocksdb/pull/10370 Test Plan: CI * Run basic db_bench for universal compaction manually Reviewed By: siying Differential Revision: D37892338 Pulled By: jay-zhuang fbshipit-source-id: 792bbd91b1ccc2f62b5d14c53118434bcaac4bbe |
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Jay Zhuang
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a3acf2ef87 |
Add seqno to time mapping (#10338)
Summary: Which will be used for tiered storage to preclude hot data from compacting to the cold tier (the last level). Internally, adding seqno to time mapping. A periodic_task is scheduled to record the current_seqno -> current_time in certain cadence. When memtable flush, the mapping informaiton is stored in sstable property. During compaction, the mapping information are merged and get the approximate time of sequence number, which is used to determine if a key is recently inserted or not and preclude it from the last level if it's recently inserted (within the `preclude_last_level_data_seconds`). Pull Request resolved: https://github.com/facebook/rocksdb/pull/10338 Test Plan: CI Reviewed By: siying Differential Revision: D37810187 Pulled By: jay-zhuang fbshipit-source-id: 6953be7a18a99de8b1cb3b162d712f79c2b4899f |