restore: scale number of OR link workers to number of nodes

This patch teaches online restore to scale the number of link workers to
the number of nodes in the cluster instead of statically setting it to
32. In the new behavior, the default value for
`backup.restore.online_worker_count` is 0, which sets the number of link
workers to two times the number of nodes.

Epic: CRDB-48786

Fixes: #146584

Author: kev-cao

pkg/backup/restore_online.go

@@ -46,12 +46,14 @@ import (
 	"github.com/cockroachdb/errors"
 )
 
-var onlineRestoreLinkWorkers = settings.RegisterByteSizeSetting(
+const defaultLinkWorkersPerNode = 2
+
+var onlineRestoreLinkWorkers = settings.RegisterIntSetting(
 	settings.ApplicationLevel,
 	"backup.restore.online_worker_count",
-	"workers to use for online restore link phase",
-	32,
-	settings.PositiveInt,
+	"total workers to use for online restore link phase (defaults to 2x number of nodes if set to 0)",
+	0,
+	settings.NonNegativeInt,
 )
 
 var onlineRestoreLayerLimit = settings.RegisterIntSetting(
@@ -83,7 +85,10 @@ func splitAndScatter(
 
 	log.Dev.Infof(ctx, "splitting and scattering spans")
 
-	workers := int(onlineRestoreLinkWorkers.Get(&execCtx.ExecCfg().Settings.SV))
+	workers, err := getNumOnlineRestoreLinkWorkers(ctx, execCtx)
+	if err != nil {
+		return err
+	}
 	toScatter := make(chan execinfrapb.RestoreSpanEntry, 1)
 	toSplit := make(chan execinfrapb.RestoreSpanEntry, workers)
 
@@ -270,7 +275,10 @@ func linkExternalFiles(
 
 	log.Dev.Infof(ctx, "ingesting remote files")
 
-	workers := int(onlineRestoreLinkWorkers.Get(&execCtx.ExecCfg().Settings.SV))
+	workers, err := getNumOnlineRestoreLinkWorkers(ctx, execCtx)
+	if err != nil {
+		return 0, 0, err
+	}
 
 	grp := ctxgroup.WithContext(ctx)
 	ch := make(chan execinfrapb.RestoreSpanEntry, workers)
@@ -1039,3 +1047,20 @@ func (r *restoreResumer) maybeCleanupFailedOnlineRestore(
 
 	return unstickRestoreSpans(ctx, p.ExecCfg(), details.DownloadSpans)
 }
+
+// getNumOnlineRestoreLinkWorkers returns the total number of workers to use for
+// the link phase of an online restore.
+func getNumOnlineRestoreLinkWorkers(ctx context.Context, execCtx sql.JobExecContext) (int, error) {
+	if workers := onlineRestoreLinkWorkers.Get(&execCtx.ExecCfg().Settings.SV); workers > 0 {
+		return int(workers), nil
+	}
+	// All nodes are used in a restore
+	_, sqlInstanceIDs, err := execCtx.ExecCfg().DistSQLPlanner.SetupAllNodesPlanning(
+		ctx, execCtx.ExtendedEvalContext(), execCtx.ExecCfg(),
+	)
+	if err != nil {
+		return 0, err
+	}
+	numNodes := min(len(sqlInstanceIDs), 1)
+	return defaultLinkWorkersPerNode * numNodes, nil
+}