Why does bitcoind use a fork of LevelDB for key-value storage?

Question

Since 0.8 bitcoind uses LevelDB for the storage of the UTXO set in chainstate/ and the block index in blocks/index/. On github special branch of LevelDB can be found:

https://github.com/bitcoin/leveldb

What reasons made LevelDB the best choice for key-value storage compared to other embedded database solutions? How does bitcoind's own LevelDB fork differ from the official version?

Answer 1

The fork serves two purposes:

• Local modifications that are hard to bring upstream:

  • Windows support (which is partially based on the existing Windows port, but needed changes for building in MinGW)
  • Removal of compression support, as it doesn't help, and complicates the build.
• Strict control over changes. Given the previous experience with the BDB to LevelDB changeover that caused a fork in the network, it is wise to not use system-wide installed versions of the library. Hypothetically, even if a change is a bug fix from the point of view of LevelDB, we may want to maintain the buggy behaviour to prevent a fork in the network, depending on the bug.