a033e280e6
* Mas i370 patch d (#383) * Refactor penciller memory In high-volume tests on large key-count clusters, so significant variation in the P0031 time has been seen: TimeBucket PatchA a.0ms_to_1ms 18554 b.1ms_to_2ms 51778 c.2ms_to_3ms 696 d.3ms_to_5ms 220 e.5ms_to_8ms 59 f.8ms_to_13ms 40 g.13ms_to_21ms 364 h.21ms_to_34ms 277 i.34ms_to_55ms 34 j.55ms_to_89ms 17 k.89ms_to_144ms 21 l.144ms_to_233ms 31 m.233ms_to_377ms 45 n.377ms_to_610ms 52 o.610ms_to_987ms 59 p.987ms_to_1597ms 55 q.1597ms_to_2684ms 54 r.2684ms_to_4281ms 29 s.4281ms_to_6965ms 7 t.6295ms_to_11246ms 1 It is unclear why this varies so much. The time to add to the cache appears to be minimal (but perhaps there is an issue with timing points in the code), whereas the time to add to the index is much more significant and variable. There is also variable time when the memory is rolled (although the actual activity here appears to be minimal. The refactoring here is two-fold: - tidy and simplify by keeping LoopState managed within handle_call, and add more helpful dialyzer specs; - change the update to the index to be a simple extension of a list, rather than any conversion. This alternative version of the pmem index in unit test is orders of magnitude faster to add - and is the same order of magnitude to check. Anticipation is that it may be more efficient in terms of memory changes. * Compress SST index Reduces the size of the leveled_sst index with two changes: 1 - Where there is a common prefix of tuple elements (e.g. Bucket) across the whole leveled_sst file - only the non-common part is indexed, and a function is used to compare. 2 - There is less "indexing" of the index i.e. only 1 in 16 keys are passed into the gb_trees part instead of 1 in 4 * Immediate hibernate Reasons for delay in hibernate were not clear. Straight after creation the process will not be in receipt of messages (must wait for the manifest to be updated), so better to hibernate now. This also means the log PC023 provides more accurate information. * Refactor BIC This patch avoids the following: - repeated replacement of the same element in the BIC (via get_kvrange), by checking presence via GET before sing SET - Stops re-reading of all elements to discover high modified date Also there appears to have been a bug where a missing HMD for the file is required to add to the cache. However, now the cache may be erased without erasing the HMD. This means that the cache can never be rebuilt * Use correct size in test results erts_debug:flat_size/1 returns size in words (i.e. 8 bytes on 64-bit CPU) not bytes * Don't change summary record As it is persisted as part of the file write, any change to the summary record cannot be rolled back * Clerk to prompt L0 write Simplifies the logic if the clerk request work for the penciller prompts L0 writes as well as Manifest changes. The advantage now is that if the penciller memory is full, and PUT load stops, the clerk should still be able to prompt persistence. the penciller can therefore make use of dead time this way * Add push on journal compact If there has been a backlog, followed by a quiet period - there may be a large ledger cache left unpushed. Journal compaction events are about once per hour, so the performance overhead of a false push should be minimal, with the advantage of clearing any backlog before load starts again. This is only relevant to riak users with very off/full batch type workloads. * Extend tests To more consistently trigger all overload scenarios * Fix range keys smaller than prefix Can't make end key an empty binary in this case, as it may be bigger than any keys within the range, but will appear to be smaller. Unit tests and ct tests added to expose the potential issue * Tidy-up - Remove penciller logs which are no longer called - Get pclerk to only wait MIN_TIMEOUT after doing work, in case there is a backlog - Remove update_levelzero_cache function as it is unique to handle_call of push_mem, and simple enough to be inline - Alight testutil slow offer with standard slow offer used * Tidy-up Remove pre-otp20 references. Reinstate the check that the starting pid is still active, this was added to tidy up shutdown. Resolve failure to run on otp20 due to `-if` sttaement * Tidy up Using null rather then {null, Key} is potentially clearer as it is not a concern what they Key is in this case, and removes a comparison step from the leveled_codec:endkey_passed/2 function. There were issues with coverage in eunit tests as the leveled_pclerk shut down. This prompted a general tidy of leveled_pclerk (remove passing of LoopState into internal functions, and add dialyzer specs. * Remove R16 relic * Further testing another issue The StartKey must always be less than or equal to the prefix when the first N characters are stripped, but this is not true of the EndKey (for the query) which does not have to be between the FirstKey and the LastKey. If the EndKey query does not match it must be greater than the Prefix (as otherwise it would not have been greater than the FirstKey - so set to null. * Fix unit test Unit test had a typo - and result interpretation had a misunderstanding. * Code and spec tidy Also look to the cover the situation when the FirstKey is the same as the Prefix with tests. This is, in theory, not an issue as it is the EndKey for each sublist which is indexed in leveled_tree. However, guard against it mapping to null here, just in case there are dangers lurking (note that tests will still pass without `M > N` guard in place. * Hibernate on BIC complete There are three situations when the BIC becomes complete: - In a file created as part of a merge the BIS is learned in the merge - After startup, files below L1 learn the block cache through reads that happen to read the block, eventually the while cache will be read, unless... - Either before/after the cache is complete, it can get whiped by a timeout after a get_sqn request (e.g. as prompted by a journal compaction) ... it will then be re-filled of the back of get/get-range requests. In all these situations we want to hibernate after the BIC is fill - to reflect the fact that the LoopState should now be relatively stable, so it is a good point to GC and rationalise location of data. Previously on the the first base was covered. Now all three are covered through the bic_complete message. * Test all index keys have same term This works functionally, but is not optimised (the term is replicated in the index) * Summaries with same index term If the summary index all have the same index term - only the object keys need to be indexes * Simplify case statements We either match the pattern of <<Prefix:N, Suffix>> or the answer should be null * OK for M == N If M = N for the first key, it will have a suffix of <<>>. This will match (as expected) a query Start Key of the sam size, and be smaller than any query Start Key that has the same prefix. If the query Start Key does not match the prefix - it will be null - as it must be smaller than the Prefix (as other wise the query Start Key would be bigger than the Last Key). The constraint of M > N was introduced before the *_prefix_filter functions were checking the prefix, to avoid issues. Now the prefix is being checked, then M == N is ok. * Simplify Correct the test to use a binary field in the range. To avoid further issue, only apply filter when everything is a binary() type. * Add test for head_only mode When leveled is used as a tictacaae key store (in parallel mode), the keys will be head_only entries. Double check they are handled as expected like object keys * Revert previous change - must support typed buckets Add assertion to confirm worthwhile optimisation * Add support for configurable cache multiple (#375) * Mas i370 patch e (#385) Improvement to monitoring for efficiency and improved readability of logs and stats. As part of this, where possible, tried to avoid updating loop state on READ messages in leveled processes (as was the case when tracking stats within each process). No performance benefits found with change, but improved stats has helped discover other potential gains. |
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Leveled - An Erlang Key-Value Store
Introduction
Leveled is a simple Key-Value store based on the concept of Log-Structured Merge Trees, with the following characteristics:
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Optimised for workloads with larger values (e.g. > 4KB).
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Explicitly supports HEAD requests in addition to GET requests:
- Splits the storage of value between keys/metadata and body (assuming some definition of metadata is provided);
- Allows for the application to define what constitutes object metadata and what constitutes the body (value-part) of the object - and assign tags to objects to manage multiple object-types with different extraction rules;
- Stores keys/metadata in a merge tree and the full object in a journal of CDB files
- allowing for HEAD requests which have lower overheads than GET requests; and
- queries which traverse keys/metadatas to be supported with fewer side effects on the page cache than folds over keys/objects.
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Support for tagging of object types and the implementation of alternative store behaviour based on type.
- Allows for changes to extract specific information as metadata to be returned from HEAD requests;
- Potentially usable for objects with special retention or merge properties.
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Support for low-cost clones without locking to provide for scanning queries (e.g. secondary indexes).
- Low cost specifically where there is a need to scan across keys and metadata (not values).
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Written in Erlang as a message passing system between Actors.
The store has been developed with a focus on being a potential backend to a Riak KV database, rather than as a generic store. It is intended to be a fully-featured backend - including support for secondary indexes, multiple fold types and auto-expiry of objects.
An optimised version of Riak KV has been produced in parallel which will exploit the availability of HEAD requests (to access object metadata including version vectors), where a full GET is not required. This, along with reduced write amplification when compared to leveldb, is expected to offer significant improvement in the volume and predictability of throughput for workloads with larger (> 4KB) object sizes, as well as reduced tail latency.
There may be more general uses of Leveled, with the following caveats:
- Leveled should be extended to define new tags that specify what metadata is to be extracted for the inserted objects (or to override the behaviour for the ?STD_TAG). Without this, there will be limited scope to take advantage of the relative efficiency of HEAD and FOLD_HEAD requests.
- If objects are small, the
head_onlymode may be used, which will cease separation of object body from header and use the Key/Metadata store as the only long-term persisted store. In this mode all of the object is treated as Metadata, and the behaviour is closer to that of the leveldb LSM-tree, although with higher median latency.
More Details
For more details on the store:
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An introduction to Leveled covers some context to the factors motivating design trade-offs in the store.
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The design overview explains the actor model used and the basic flow of requests through the store.
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Future work covers new features being implemented at present, and improvements necessary to make the system production ready.
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There is also a "Why" section looking at lower level design choices and the rationale that supports them.
Feedback
Please create an issue if you have any suggestions. You can ping me @masleeds if you wish
Running Leveled
Unit and current tests in leveled should run with rebar3.
A new database can be started by running
{ok, Bookie} = leveled_bookie:book_start(StartupOptions)
This will start a new Bookie. It will start and look for existing data files, under the RootPath, and start empty if none exist. Further information on startup options can be found here here.
The book_start method should respond once startup is complete. The leveled_bookie module includes the full API for external use of the store.
Running in Riak requires Riak 2.9 or beyond, which is available from January 2019.
There are three main branches:
develop-3.1 - default: Target for the Riak 3.1 release with support for OTP 22 and OTP 24;
develop-3.0: Used in the Riak 3.0 release with support for OTP 20 and OTP 22;
develop-2.9: Used in the Riak 2.9 release with support for OTP R16 through to OTP 20.
Contributing
In order to contribute to leveled, fork the repository, make a branch for your changes, and open a pull request. The acceptance criteria for updating leveled is that it passes rebar3 dialyzer, xref, eunit, and ct with 100% coverage.
To have rebar3 execute the full set of tests, run:
rebar3 as test do xref, dialyzer, cover --reset, eunit --cover, ct --cover, cover --verbose
For those with a Quickcheck license, property-based tests can also be run using:
rebar3 as eqc do eunit --module=leveled_simpleeqc, eunit --module=leveled_statemeqc