leveldb-Impl：VersionSet.java

public class VersionSet
        implements SeekingIterable<InternalKey, Slice>
{
    private static final int L0_COMPACTION_TRIGGER = 4;

    public static final int TARGET_FILE_SIZE = 2 * 1048576;

    // Maximum bytes of overlaps in grandparent (i.e., level+2) before we
    // stop building a single file in a level.level+1 compaction.
    public static final long MAX_GRAND_PARENT_OVERLAP_BYTES = 10 * TARGET_FILE_SIZE;

    private final AtomicLong nextFileNumber = new AtomicLong(2);
    private long manifestFileNumber = 1;
    private Version current;
    private long lastSequence;
    private long logNumber;
    private long prevLogNumber;

    private final Map<Version, Object> activeVersions = new MapMaker().weakKeys().makeMap();
    private final File databaseDir;
    private final TableCache tableCache;
    private final InternalKeyComparator internalKeyComparator;

    private LogWriter descriptorLog;
    private final Map<Integer, InternalKey> compactPointers = new TreeMap<>();