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-rw-r--r--doc/manual/opt-common-syn.xml1
-rw-r--r--doc/manual/opt-common.xml25
2 files changed, 26 insertions, 0 deletions
diff --git a/doc/manual/opt-common-syn.xml b/doc/manual/opt-common-syn.xml
index f9004186355d..f848ad249909 100644
--- a/doc/manual/opt-common-syn.xml
+++ b/doc/manual/opt-common-syn.xml
@@ -15,3 +15,4 @@
 <arg><option>-k</option></arg>
 <arg><option>--keep-failed</option></arg>
 <arg><option>-K</option></arg>
+<arg><option>--fallback</option></arg>
diff --git a/doc/manual/opt-common.xml b/doc/manual/opt-common.xml
index 158e282fdb55..db8a60ecad6e 100644
--- a/doc/manual/opt-common.xml
+++ b/doc/manual/opt-common.xml
@@ -147,3 +147,28 @@
   </listitem>
 </varlistentry>
 
+
+<varlistentry>
+  <term><option>--fallback</option></term>
+  <listitem>
+    <para>
+      Whenever Nix attempts to realise a derivation for which a
+      closure is already known, but this closure cannot be realised,
+      fall back on normalising the derivation.
+    </para>
+
+    <para>
+      The most common scenario in which this is useful is when we have
+      registered substitutes in order to perform binary distribution
+      from, say, a network repository.  If the repository is down, the
+      realisation of the derivation will fail.  When this option is
+      specified, Nix will build the derivation instead.  Thus,
+      binary installation falls back on a source installation.  This
+      option is not the default since it is generally not desirable
+      for a transient failure in obtaining the substitutes to lead to
+      a full build from source (with the related consumption of
+      resources).
+    </para>
+  </listitem>
+</varlistentry>
+