sftp: allow customizing MaxConcurrentRequestsPerFile and MaxPacket

changelog/unreleased/pull-4445

@@ -0,0 +1,9 @@
+Enhancement: allow customizing MaxConcurrentRequestsPerFile and MaxPacket parameters in the sftp backend
+
+SFTP over long fat links suffers from poor performance due to its by default
+overly small max payload size. But implementations such as OpenSSH tend to
+accept vastly bigger packets, which improves backup performance for this
+kind of situation. Restic now allows customizing the max packet size and
+max concurrent requests per file parameters in the sftp backend.
+
+https://github.com/restic/restic/pull/4445

internal/backend/sftp/config.go

@@ -16,13 +16,17 @@ type Config struct {
 	Layout  string `option:"layout" help:"use this backend directory layout (default: auto-detect)"`
 	Command string `option:"command" help:"specify command to create sftp connection"`
 
-	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
+	Connections                  uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
+	MaxConcurrentRequestsPerFile int  `option:"max_concurrent_requests_per_file" help:"sets the maximum concurrent requests allowed for a single file (default: 64)"`
+	MaxPacket                    int  `option:"max_packet" help:"sets the maximum size of the payload, measured in bytes (default: 32768)"`
 }
 
 // NewConfig returns a new config with default options applied.
 func NewConfig() Config {
 	return Config{
-		Connections: 5,
+		Connections:                  5,
+		MaxConcurrentRequestsPerFile: 64,
+		MaxPacket:                    32768,
 	}
 }
 

internal/backend/sftp/sftp.go

@@ -102,7 +102,10 @@ func startClient(cfg Config) (*SFTP, error) {
 	}()
 
 	// open the SFTP session
-	client, err := sftp.NewClientPipe(rd, wr)
+	client, err := sftp.NewClientPipe(rd, wr,
+		sftp.MaxConcurrentRequestsPerFile(cfg.MaxConcurrentRequestsPerFile),
+		sftp.MaxPacketUnchecked(cfg.MaxPacket),
+	)
 	if err != nil {
 		return nil, errors.Errorf("unable to start the sftp session, error: %v", err)
 	}