51 lines
2.3 KiB
Markdown
51 lines
2.3 KiB
Markdown
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---
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title: Bulkoading by ingesting external SST files
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layout: post
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author: IslamAbdelRahman
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category: blog
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---
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## Introduction
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One of the basic operations of RocksDB is writing to RocksDB, Writes happen when user call (DB::Put, DB::Write, DB::Delete ... ), but what happens when you write to RocksDB ? .. this is a brief description of what happens.
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- User insert a new key/value by calling DB::Put() (or DB::Write())
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- We create a new entry for the new key/value in our in-memory structure (memtable / SkipList by default) and we assign it a new sequence number.
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- When the memtable exceeds a specific size (64 MB for example), we convert this memtable to a SST file, and put this file in level 0 of our LSM-Tree
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- Later, compaction will kick in and move data from level 0 to level 1, and then from level 1 to level 2 .. and so on
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But what if we can skip these steps and add data to the lowest possible level directly ? This is what bulk-loading does
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## Bulkloading
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- Write all of our keys and values into SST file outside of the DB
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- Add the SST file into the LSM directly
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This is bulk-loading, and in specific use-cases it allow users to achieve faster data loading and better write-amplification.
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and doing it is as simple as
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```cpp
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Options options;
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SstFileWriter sst_file_writer(EnvOptions(), options, options.comparator);
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Status s = sst_file_writer.Open(file_path);
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assert(s.ok());
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// Insert rows into the SST file, note that inserted keys must be
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// strictly increasing (based on options.comparator)
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for (...) {
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s = sst_file_writer.Add(key, value);
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assert(s.ok());
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}
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// Ingest the external SST file into the DB
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s = db_->IngestExternalFile({"/home/usr/file1.sst"}, IngestExternalFileOptions());
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assert(s.ok());
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```
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You can find more details about how to generate SST files and ingesting them into RocksDB in this [wiki page](https://github.com/facebook/rocksdb/wiki/Creating-and-Ingesting-SST-files)
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## Use cases
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There are multiple use cases where bulkloading could be useful, for example
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- Generating SST files in offline jobs in Hadoop, then downloading and ingesting the SST files into RocksDB
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- Migrating shards between machines by dumping key-range in SST File and loading the file in a different machine
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- Migrating from a different storage (InnoDB to RocksDB migration in MyRocks)
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