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rolla

rolla is a simple key/value storage engine. It aims to achieve a good balance of fast query performance, low disk space, and data safety while being stingy with memory. It was designed primarily for embedding in mobile applications.

Features

  • Database is pure C and relies only on the C standard library
  • Database is crash-only design; database files cannot go corrupt
  • Inserts are O(1)
  • Queries are O(1) .. O(n), though on typical workloads with hot spots the performance is closer to O(1) due to scanning hash buckets in write-recency order
  • An idle (but fully loaded) database uses < 40kB of memory with default tuning parameters
  • Database files can compact on close, so amortized disk use is O(keys)
  • A key enumeration interface is supported
  • On-disk representation has only 10 bytes of overhead for each key/value pair
  • Already in use on millions of iOS and Android devices (armv7)

Caveats

  • rolla databases are NOT THREAD SAFE. If you want to access a rolla database from multiple threads concurrently, you'll have to serialize access in a higher layer. The library is fully re-entrant, however--so you can use different databases in multiple threads concurrently.
  • Keys are null-terminated C-Strings, not bytestrings
  • Keys must be <=255 bytes
  • An entire database cannot exceed 4GB
  • Misses are expensive (they must read approx 1/8192 of the database off disk to invalidate the key)
  • Compaction only happens when the database is closed and reloaded, it cannot be done while online
  • Files are assumed little endian and are not portable across architectures of different byte orderings
  • rolla does not call fsync() or msync(), but instead relies on the operating system's schedule. Some data that was not totally committed to the backing hardware could be lost if system failure happened (though the database will come up cleanly with a subset of records, "repair" is never necessary). Applications can call rolla_sync() if they want to force msync more often.
  • A database created with a certain bucket count must always be used with that bucket count. The bucket count cannot change.

Design

Rolla has a very simple design.

When a rolla database is created, an array of buckets is allocated (by default 8192). Database keys are hashed onto these buckets. The bucket values contain the offset to the last entry in the file on disk that contains a key that was hashed into that bucket.

On a read, this last value is checked. If the key matches, the value is returned; otherwise, the record at this offset in the file contains a link back to the previous offset in this bucket, and so on. Traversal continues until a record is encountered with a sentinel value that indicates there are no more records in this bucket. This is why misses (key not found) are expensive in rolla.

A write is simply a matter of overwriting the in-memory bucket value to point to the current EOF marker in the on-disk file, then taking the old value from this bucket and writing it out to the disk as the previous link in the chain, along with the new key and value. In this way, writes are always appends.

Mutating the same key over and over again will grow the database without bound. rolla_close() takes a compress flag that will walk the database and write a new version with only the current values for each key. It is important to close and compress active rolla databases regularly to both preserve disk space and prevent them from exceeding their 4GB limit. Compaction is done in a crash-safe manner using a tempfile and rename().

When a database is reloaded, the file is walked and the bucket array is re-populated with the final offsets. Any partial records are truncated in case of a crash.

Rolla uses the mmap() syscall to memory-map this file as an array of records (C structs).

Moving the NUMBUCKETS constant up and down can tune rolla for different expected key counts. Clearly, larger bucket counts result in fewer seeks on disk because only a very few keys will map to that bucket, and link walking is minimal or absent.

Rolla was designed to run on 32-bit systems, so offset values are typically unsigned 32-bit integers. This limits total database size to ~4GB.

Usage

You can build a quick rolla_test program using redo. Install this:

https://github.com/apenwarr/redo

... then type redo in this repository root.

But generally, just build rolla.c into your project. Recommended compiler flags are:

CFLAGS="-g -Wall -Werror -pedantic -std=gnu99 -O2 -fno-strict-aliasing"

See rolla.h for API. set, get, del, sync, create, close, iter. It's a key/value storage engine!

Iteration Note

If you change values (set or del) in rolla_iter(), make sure to pass copies of keys to set and del. The keys given to you by rolla are being borrowed from the mmaped file, and if the memory mapped region should move due to database growth caused by your mutation, those pointers will become invalid.

Benchmarks

Hardware

VMWare Fusion
Arch Linux 3.3.7-1-ARCH #1 SMP PREEMPT x64
Intel(R) Core(TM) i7-2677M CPU @ 1.80GHz
Crucial 256GB SSD
1.4GB RAM (likely on-disk file fully in page cache)

Data

Keys and values are 1-6 bytes [1..1000000].

Test 1

NUMBUCKETS=8192
100,000 writes, and 100,000 reads
writes 3609742.327/s
reads  4034846.852/s
(note: this is the sweet spot for this library)

Test 2

NUMBUCKETS=8192
1,000,000 writes, and 1,000,000 reads
writes 5558771.063/s
reads   621547.615/s
(note: too few buckets, too much chaining)

Test 3

NUMBUCKETS=262144
1,000,000 writes, and 1,000,000 reads
writes 4818579.943/s
reads  3342098.868/s
(note: less chaining and more req'd ram.. ~1MB)

Author

Jamie Turner [email protected] and others at Bump Technologies, Inc.