docs+lncfg+sample-lnd.conf: add no-graph-cache option

docs/release-notes/release-notes-0.14.0.md

@@ -92,6 +92,14 @@ usage. Users running `lnd` on low-memory systems are advised to run with the
 removes zombie channels from the graph, reducing the number of channels that
 need to be kept in memory.
 
+There is a [fallback option](https://github.com/lightningnetwork/lnd/pull/5840)
+`db.no-graph-cache=true` that can be used when running a Bolt (`bbolt`) based
+database backend. Using the database for path finding is considerably slower
+than using the in-memory graph cache but uses less RAM. The fallback option is
+not available for `etcd` or Postgres database backends because of the way they
+handle long-running database transactions that are required for the path finding
+operations.
+
 ## Protocol Extensions
 
 ### Explicit Channel Negotiation

lncfg/db.go

@@ -57,6 +57,8 @@ type DB struct {
 	Bolt *kvdb.BoltConfig `group:"bolt" namespace:"bolt" description:"Bolt settings."`
 
 	Postgres *postgres.Config `group:"postgres" namespace:"postgres" description:"Postgres settings."`
+
+	NoGraphCache bool `long:"no-graph-cache" description:"Don't use the in-memory graph cache for path finding. Much slower but uses less RAM. Can only be used with a bolt database backend."`
 }
 
 // DefaultDB creates and returns a new default DB config.
@@ -87,8 +89,21 @@ func (db *DB) Validate() error {
 		}
 
 	default:
-		return fmt.Errorf("unknown backend, must be either \"%v\" or \"%v\"",
-			BoltBackend, EtcdBackend)
+		return fmt.Errorf("unknown backend, must be either '%v' or "+
+			"'%v'", BoltBackend, EtcdBackend)
+	}
+
+	// The path finding uses a manual read transaction that's open for a
+	// potentially long time. That works fine with the locking model of
+	// bbolt but can lead to locks or rolled back transactions with etcd or
+	// postgres. And since we already have a smaller memory footprint for
+	// remote database setups (due to not needing to memory-map the bbolt DB
+	// files), we can keep the graph in memory instead. But for mobile
+	// devices the tradeoff between a smaller memory footprint and the
+	// longer time needed for path finding might be a desirable one.
+	if db.NoGraphCache && db.Backend != BoltBackend {
+		return fmt.Errorf("cannot use no-graph-cache with database "+
+			"backend '%v'", db.Backend)
 	}
 
 	return nil

sample-lnd.conf

@@ -1118,6 +1118,9 @@ litecoin.node=ltcd
 ; a batch of modifications to disk. Defaults to 500 milliseconds.
 ; db.batch-commit-interval=500ms
 
+; Don't use the in-memory graph cache for path finding. Much slower but uses
+; less RAM. Can only be used with a bolt database backend.
+; db.no-graph-cache=true
 
 [etcd]
 
@@ -1170,6 +1173,27 @@ litecoin.node=ltcd
 ; disable.
 ; db.postgres.timeout=
 
+[bolt]
+
+; If true, prevents the database from syncing its freelist to disk. 
+; db.bolt.nofreelistsync=1
+
+; Whether the databases used within lnd should automatically be compacted on
+; every startup (and if the database has the configured minimum age). This is
+; disabled by default because it requires additional disk space to be available
+; during the compaction that is freed afterwards. In general compaction leads to
+; smaller database files.
+; db.bolt.auto-compact=true
+
+; How long ago the last compaction of a database file must be for it to be
+; considered for auto compaction again. Can be set to 0 to compact on every
+; startup. (default: 168h)
+; db.bolt.auto-compact-min-age=0
+
+; Specify the timeout to be used when opening the database.
+; db.bolt.dbtimeout=60s
+
+
 [cluster]
 
 ; Enables leader election if set.
@@ -1217,26 +1241,6 @@ litecoin.node=ltcd
 ; The TLS certificate to use for establishing the remote signer's identity.
 ; remotesigner.tlscertpath=/path/to/remote/signer/tls.cert
 
-[bolt]
-
-; If true, prevents the database from syncing its freelist to disk. 
-; db.bolt.nofreelistsync=1
-
-; Whether the databases used within lnd should automatically be compacted on
-; every startup (and if the database has the configured minimum age). This is
-; disabled by default because it requires additional disk space to be available
-; during the compaction that is freed afterwards. In general compaction leads to
-; smaller database files.
-; db.bolt.auto-compact=true
-
-; How long ago the last compaction of a database file must be for it to be
-; considered for auto compaction again. Can be set to 0 to compact on every
-; startup. (default: 168h)
-; db.bolt.auto-compact-min-age=0
-
-; Specify the timeout to be used when opening the database.
-; db.bolt.dbtimeout=60s
-
 
 [gossip]