Leveled

leveled

Leveled is a simple Key-Value store based on the concept of Log-Structured Merge Trees, with the following characteristics:

  • Optimised for workloads with larger values (e.g. > 4KB).
  • 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.
  • 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.
  • 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).
  • Written in Erlang as a message passing system between Actors.

Strengths

  1. leveled was developed specifically as a potential backend for Riak, with features such as:
    • Support for secondary indexes
    • Multiple fold types
    • Auto expiry of objects Enabling compression means more CPU usage but less disk space. Compression is especially good for text data, including raw text, Base64, JSON, etc.
  2. Optimised for workloads with larger values (e.g. > 4KB).
  3. Explicitly supports HEAD requests in addition to GET requests.
  4. Support for low-cost clones without locking to provide for scanning queries (e.g. secondary indexes).

Weaknesses

  1. Leveled is still a comparatively new technology and more likely to suffer from edge case issues than Bitcask or LevelDB simply because they’ve been around longer and have been more thoroughly tested via usage in customer environments.
  2. Leveled works better with medium to larger sized objects. It works perfectly well with small objects but the additional diskspace overhead may render LevelDB a better choice if disk space is at a premium and all of your data will be exclusively limited a few KB or less. This may change as Leveled matures though.

Installing leveled

Leveled is included with Riak KV 2.9.10 and beyond, so there is no need to install anything further.

storage_backend = leveled

{riak_kv, [
    %% ...
    {storage_backend, riak_kv_leveled_backend},
    %% ...
    ]}

Configuring leveled

Leveled’s default behavior can be modified by adding/changing parameters in the leveled section of the riak.conf. The section below details the parameters you’ll use to modify leveled.

The configuration values that can be set in your riak.conf for leveled are as follows:

Config Description Default
leveled.data_root leveled data root. ./data/leveled
leveled.sync_strategy Strategy for flushing data to disk. none
leveled.compression_method Compression Method. native
leveled.compression_point Compression Point - The point at which compression is applied to the Journal. on_receipt
leveled.log_level Log Level - Set the minimum log level to be used within leveled. info
leveled.journal_size The approximate size (in bytes) when a Journal file should be rolled. 1000000000
leveled.compaction_runs_perday The number of journal compactions per vnode per day 24
leveled.compaction_low_hour The hour of the day in which journal compaction can start. 0
leveled.compaction_top_hour The hour of the day, after which journal compaction should stop. 23
leveled.max_run_length Max Journal Files Per Compaction Run. 4

Below are general configuration recommendations for Linux distributions. Individual users may need to tailor these settings for their application.

sysctl

For production environments, please see System Performance Tuning for the recommended /etc/sysctl.conf settings.

Block Device Scheduler

Beginning with the 2.6 kernel, Linux gives you a choice of four I/O elevator models. We recommend using the NOOP elevator. You can do this by changing the scheduler on the Linux boot line: elevator=noop.

No Entropy

If you are using https protocol, the 2.6 kernel is widely known for stalling programs waiting for SSL entropy bits. If you are using https, we recommend installing the HAVEGE package for pseudorandom number generation.

clocksource

We recommend setting clocksource=hpet on your Linux kernel’s boot line. The TSC clocksource has been identified to cause issues on machines with multiple physical processors and/or CPU throttling.

swappiness

We recommend setting vm.swappiness=0 in /etc/sysctl.conf. The vm.swappiness default is 60, which is aimed toward laptop users with application windows. This was a key change for MySQL servers and is often referenced in database performance literature.

Implementation Details

Leveled is an open source project that has been developed specifically as a backend option for Riak, rather than a generic backend.