segments_N 和segments.gen
The active segments in the index are stored in the segment info file, segments_N. There may be one or more segments_N files in the index; however, the one with the largest generation is the active one (when older segments_N files are present it's because they temporarily cannot be deleted, or, a writer is in the process of committing, or a custom IndexDeletionPolicy is in use). This file lists each segment by name, has details about the separate norms and deletion files, and also contains the size of each segment.
As of 2.1, there is also a file segments.gen. This file contains the current generation (the _N in segments_N) of the index. This is used only as a fallback in case the current generation cannot be accurately determined by directory listing alone (as is the case for some NFS clients with time-based directory cache expiraation). This file simply contains an Int32 version header (SegmentInfos.FORMAT_LOCKLESS = -2), followed by the generation recorded as Int64, written twice.
Pre-2.1: Segments --> Format, Version, NameCounter, SegCount, <SegName, SegSize> SegCount
2.1 and above: Segments --> Format, Version, NameCounter, SegCount, <SegName, SegSize, DelGen, HasSingleNormFile, NumField, NormGenNumField, IsCompoundFile>SegCount
2.3 and above: Segments --> Format, Version, NameCounter, SegCount, <SegName, SegSize, DelGen, DocStoreOffset, [DocStoreSegment, DocStoreIsCompoundFile], HasSingleNormFile, NumField, NormGenNumField, IsCompoundFile>SegCount
Format, NameCounter, SegCount, SegSize, NumField, DocStoreOffset --> Int32
Version, DelGen, NormGen --> Int64
SegName, DocStoreSegment --> String
IsCompoundFile, HasSingleNormFile, DocStoreIsCompoundFile --> Int8
Format is -1 as of Lucene 1.4, -3 (SegmentInfos.FORMAT_SINGLE_NORM_FILE) as of Lucene 2.1 and 2.2, and -4 (SegmentInfos.FORMAT_SHARED_DOC_STORE) as of Lucene 2.3
Version counts how often the index has been changed by adding or deleting documents.
NameCounter is used to generate names for new segment files.
SegName is the name of the segment, and is used as the file name prefix for all of the files that compose the segment's index.
SegSize is the number of documents contained in the segment index.
DelGen is the generation count of the separate deletes file. If this is -1, there are no separate deletes. If it is 0, this is a pre-2.1 segment and you must check filesystem for the existence of _X.del. Anything above zero means there are separate deletes (_X_N.del).
NumField is the size of the array for NormGen, or -1 if there are no NormGens stored.
NormGen records the generation of the separate norms files. If NumField is -1, there are no normGens stored and they are all assumed to be 0 when the segment file was written pre-2.1 and all assumed to be -1 when the segments file is 2.1 or above. The generation then has the same meaning as delGen (above).
IsCompoundFile records whether the segment is written as a compound file or not. If this is -1, the segment is not a compound file. If it is 1, the segment is a compound file. Else it is 0, which means we check filesystem to see if _X.cfs exists.
If HasSingleNormFile is 1, then the field norms are written as a single joined file (with extension .nrm); if it is 0 then each field's norms are stored as separate .fN files. See "Normalization Factors" below for details.
DocStoreOffset, DocStoreSegment, DocStoreIsCompoundFile: If DocStoreOffset is -1, this segment has its own doc store (stored fields values and term vectors) files and DocStoreSegment and DocStoreIsCompoundFile are not stored. In this case all files for stored field values (*.fdt and *.fdx) and term vectors (*.tvf, *.tvd and *.tvx) will be stored with this segment. Otherwise, DocStoreSegment is the name of the segment that has the shared doc store files; DocStoreIsCompoundFile is 1 if that segment is stored in compound file format (as a .cfx file); and DocStoreOffset is the starting document in the shared doc store files where this segment's documents begin. In this case, this segment does not store its own doc store files but instead shares a single set of these files with other segments.