eth: don't inadvertently enable snapshots in archive nodes (#21025)

* eth: don't reassign more cache than is being made available

* eth: don't inadvertently enable snapshot in a case where --snapshot wasn't given

eth/backend.go

@@ -125,8 +125,12 @@ func New(ctx *node.ServiceContext, config *Config) (*Ethereum, error) {
 		config.Miner.GasPrice = new(big.Int).Set(DefaultConfig.Miner.GasPrice)
 	}
 	if config.NoPruning && config.TrieDirtyCache > 0 {
+		if config.SnapshotCache > 0 {
 			config.TrieCleanCache += config.TrieDirtyCache * 3 / 5
-		config.SnapshotCache += config.TrieDirtyCache * 3 / 5
+			config.SnapshotCache += config.TrieDirtyCache * 2 / 5
+		} else {
+			config.TrieCleanCache += config.TrieDirtyCache
+		}
 		config.TrieDirtyCache = 0
 	}
 	log.Info("Allocated trie memory caches", "clean", common.StorageSize(config.TrieCleanCache)*1024*1024, "dirty", common.StorageSize(config.TrieDirtyCache)*1024*1024)