1---
2title: Bulkloading by ingesting external SST files
3layout: post
4author: IslamAbdelRahman
5category: blog
6---
7
8## Introduction
9
10One 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.
11- User insert a new key/value by calling DB::Put() (or DB::Write())
12- 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.
13- 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
14- 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
15
16But what if we can skip these steps and add data to the lowest possible level directly ? This is what bulk-loading does
17
18## Bulkloading
19
20- Write all of our keys and values into SST file outside of the DB
21- Add the SST file into the LSM directly
22
23This is bulk-loading, and in specific use-cases it allow users to achieve faster data loading and better write-amplification.
24
25and doing it is as simple as
26```cpp
27Options options;
28SstFileWriter sst_file_writer(EnvOptions(), options, options.comparator);
29Status s = sst_file_writer.Open(file_path);
30assert(s.ok());
31
32// Insert rows into the SST file, note that inserted keys must be
33// strictly increasing (based on options.comparator)
34for (...) {
35  s = sst_file_writer.Add(key, value);
36  assert(s.ok());
37}
38
39// Ingest the external SST file into the DB
40s = db_->IngestExternalFile({"/home/usr/file1.sst"}, IngestExternalFileOptions());
41assert(s.ok());
42```
43
44You 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)
45
46## Use cases
47There are multiple use cases where bulkloading could be useful, for example
48- Generating SST files in offline jobs in Hadoop, then downloading and ingesting the SST files into RocksDB
49- Migrating shards between machines by dumping key-range in SST File and loading the file in a different machine
50- Migrating from a different storage (InnoDB to RocksDB migration in MyRocks)
51