Snapshots and deletion policies

An IndexSnapshot pins one committed generation so its segment files cannot be removed while a caller inspects or copies them.

Snapshot lifecycle

var snapshot = writer.CreateSnapshot();
try
{
    Console.WriteLine(
        $"Generation {snapshot.CommitGeneration}, " +
        $"{snapshot.Segments.Count} segments, " +
        $"taken at {snapshot.TakenAtUtc:O}");

    var backup = writer.BackupSnapshot(
        snapshot,
        "/srv/backups/index-" + snapshot.CommitGeneration);
}
finally
{
    writer.ReleaseSnapshot(snapshot);
}

The snapshot exposes:

Property	Meaning
CommitGeneration	Pinned segments_N generation
Segments	Defensive copy of the segment metadata
TakenAtUtc	Snapshot creation time

Always release in finally. An unreleased snapshot retains files until the writer is disposed.

sequenceDiagram
    participant App
    participant Writer as IndexWriter
    participant Policy as Deletion policy
    participant Merge as Merge scheduler

    App->>Writer: CreateSnapshot()
    Writer-->>App: Pinned generation
    Merge->>Writer: Publish merged segment
    Policy->>Writer: Select obsolete commits
    Writer->>Writer: Keep files referenced by snapshot
    App->>Writer: ReleaseSnapshot()
    Writer->>Policy: Re-evaluate deletable files

Deletion policies

IndexWriterConfig.DeletionPolicy controls which old commits remain when they are not pinned:

Policy	Behaviour	Trade-off
KeepLatestCommitPolicy	Retains the newest commit	Minimum steady-state history and disk usage
KeepLastNCommitsPolicy(n)	Retains the newest n generations	Faster local rollback at additional disk cost

var config = new IndexWriterConfig
{
    DeletionPolicy = new KeepLastNCommitsPolicy(maxCommits: 5),
};

Retaining commits is not a backup. All generations remain in the same directory and share unchanged segment files, so host or directory loss affects them together.

Merge interaction

A merge creates a replacement segment but cannot remove source files while any retained commit, active searcher, or snapshot still references them. Long-lived snapshots can therefore prevent expected disk reclamation even though indexing and merging continue normally.

Monitor snapshot duration and free disk space. Keep snapshots around a bounded copy operation, not an application session.

Choosing a policy

Use KeepLatestCommitPolicy with independent backups for most services. Use KeepLastNCommitsPolicy when a short local rollback window has operational value and disk growth is measured.

Commit count is not a time guarantee. A busy writer can produce several retained generations quickly, while a quiet writer can retain them for a long time.

See Backup and restore for a complete manifest-based copy.