Read ahead over SFTP: one connection with 32 requests in flight by default, more with restore_sftp_connections; a Storage Box gets slower with parallel readers

lib/Backend/SftpRangeFetcher.php

@@ -11,10 +11,9 @@ namespace OCA\NgBackup\Backend;
  * Reads byte ranges of files on an SFTP location ahead of use, over several connections at once,
  * into local files (StorageBackend::readAhead(), for a full restore).
  *
- * One connection is bound by the round trip and by what a single TCP connection gets over a long
- * line: a restore from a Hetzner Storage Box to Canada (RTT 122 ms) reached about 2 MiB/s, with
- * the PHP process mostly waiting. Here every connection reads its own range with DEPTH requests
- * in flight. PHP runs one thing at a time: while it waits for an answer on one connection, the
+ * A restore that reads its packs as it goes is bound by the round trip: from a Hetzner Storage
+ * Box to Canada (RTT 122 ms) it reached about 2.8 MiB/s, with the PHP process mostly waiting.
+ * Here every connection reads its own range with DEPTH requests in flight, ahead of the restore. PHP runs one thing at a time: while it waits for an answer on one connection, the
  * answers of the others arrive in the kernel's buffers, so the connections still transfer at the
  * same time.
  *
@@ -24,10 +23,17 @@ namespace OCA\NgBackup\Backend;
  * for the caller to read the usual way; a broken connection is not used again.
  */
 final class SftpRangeFetcher {
-	/** Connections used at once by a full restore. A Hetzner Storage Box allows 10 per account. */
-	public const CONNECTIONS = 4;
+	/**
+	 * Connections a full restore uses, unless the app setting restore_sftp_connections says
+	 * otherwise. One: a Hetzner Storage Box gave about 14 MiB/s on one connection with DEPTH
+	 * requests in flight, and 4-6 MiB/s on 2 or 4 (it reads from disks, and parallel readers make
+	 * them seek). On a line with packet loss to a server that handles parallel reads well, 4
+	 * connections did better (21 against 14 MiB/s).
+	 */
+	public const DEFAULT_CONNECTIONS = 1;
+	public const MAX_CONNECTIONS = 8;
 	/** READ requests in flight per connection. */
-	public const DEPTH = 16;
+	public const DEPTH = 32;
 	/** Ranges read ahead of the one the caller is at, in flight or waiting in the spool. */
 	public const AHEAD = 8;
 	/**

lib/Backend/StorageBackend.php

@@ -29,6 +29,8 @@ final class StorageBackend implements IBackend, IReadsAhead {
 	/** Seconds without a request after which an SFTP connection is checked before it is used. */
 	public const IDLE_CHECK = 60;
 	private float $lastUsed;
+	/** Connections readAhead() opens (see SftpRangeFetcher::DEFAULT_CONNECTIONS). */
+	public int $readAheadConnections = SftpRangeFetcher::DEFAULT_CONNECTIONS;
 	/** Times the storage was built anew after its connection died (for tests and diagnostics). */
 	public int $reconnects = 0;
 
@@ -244,7 +246,7 @@ final class StorageBackend implements IBackend, IReadsAhead {
 	}
 
 	/**
-	 * Over SFTP: SftpRangeFetcher, on CONNECTIONS new connections of its own (built with the
+	 * Over SFTP: SftpRangeFetcher, on readAheadConnections new connections of its own (built with the
 	 * reconnect function), so the storage's connection stays free for other requests. Elsewhere
 	 * not (yet): null.
 	 */
@@ -262,7 +264,8 @@ final class StorageBackend implements IBackend, IReadsAhead {
 		$storages = [];
 		$connections = [];
 		$root = '';
-		for ($n = 0; $n < SftpRangeFetcher::CONNECTIONS; $n++) {
+		$wanted = max(1, min(SftpRangeFetcher::MAX_CONNECTIONS, $this->readAheadConnections));
+		for ($n = 0; $n < $wanted; $n++) {
 			try {
 				$storage = $reconnect();
 				$sftp = SftpReadStream::connection($storage);