diff --git a/src/libstore/gc.cc b/src/libstore/gc.cc
index d210defe5..95a4bc934 100644
--- a/src/libstore/gc.cc
+++ b/src/libstore/gc.cc
@@ -419,7 +419,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
 
                 try {
                     auto mapFile = fmt("/proc/%s/maps", ent->d_name);
-                    auto mapLines = tokenizeString<std::vector<string>>(readFile(mapFile, true), "\n");
+                    auto mapLines = tokenizeString<std::vector<string>>(readFile(mapFile), "\n");
                     for (const auto & line : mapLines) {
                         auto match = std::smatch{};
                         if (std::regex_match(line, match, mapRegex))
@@ -427,7 +427,7 @@ void LocalStore::findRuntimeRoots(Roots & roots, bool censor)
                     }
 
                     auto envFile = fmt("/proc/%s/environ", ent->d_name);
-                    auto envString = readFile(envFile, true);
+                    auto envString = readFile(envFile);
                     auto env_end = std::sregex_iterator{};
                     for (auto i = std::sregex_iterator{envString.begin(), envString.end(), storePathRegex}; i != env_end; ++i)
                         unchecked[i->str()].emplace(envFile);
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index 5780c93a6..a04118512 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -148,6 +148,9 @@ struct StringSink : Sink
 {
     ref<std::string> s;
     StringSink() : s(make_ref<std::string>()) { };
+    explicit StringSink(const size_t reservedSize) : s(make_ref<std::string>()) {
+      s->reserve(reservedSize);
+    };
     StringSink(ref<std::string> s) : s(s) { };
     void operator () (const unsigned char * data, size_t len) override;
 };
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 615a7656c..71db92d77 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -316,19 +316,16 @@ string readFile(int fd)
     if (fstat(fd, &st) == -1)
         throw SysError("statting file");
 
-    std::vector<unsigned char> buf(st.st_size);
-    readFull(fd, buf.data(), st.st_size);
-
-    return string((char *) buf.data(), st.st_size);
+    return drainFD(fd, true, st.st_size);
 }
 
 
-string readFile(const Path & path, bool drain)
+string readFile(const Path & path)
 {
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
     if (!fd)
         throw SysError(format("opening file '%1%'") % path);
-    return drain ? drainFD(fd.get()) : readFile(fd.get());
+    return readFile(fd.get());
 }
 
 
@@ -665,9 +662,9 @@ void writeFull(int fd, const string & s, bool allowInterrupts)
 }
 
 
-string drainFD(int fd, bool block)
+string drainFD(int fd, bool block, const size_t reserveSize)
 {
-    StringSink sink;
+    StringSink sink(reserveSize);
     drainFD(fd, sink, block);
     return std::move(*sink.s);
 }
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index d7ee62bcc..1b263abcc 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -103,7 +103,7 @@ unsigned char getFileType(const Path & path);
 
 /* Read the contents of a file into a string. */
 string readFile(int fd);
-string readFile(const Path & path, bool drain = false);
+string readFile(const Path & path);
 void readFile(const Path & path, Sink & sink);
 
 /* Write a string to a file. */
@@ -162,7 +162,7 @@ MakeError(EndOfFile, Error);
 
 
 /* Read a file descriptor until EOF occurs. */
-string drainFD(int fd, bool block = true);
+string drainFD(int fd, bool block = true, const size_t reserveSize=0);
 
 void drainFD(int fd, Sink & sink, bool block = true);
 
diff --git a/src/nix/ls.cc b/src/nix/ls.cc
index 6ae4df27e..b9716a6a1 100644
--- a/src/nix/ls.cc
+++ b/src/nix/ls.cc
@@ -140,7 +140,7 @@ struct CmdLsNar : Command, MixLs
 
     void run() override
     {
-        list(makeNarAccessor(make_ref<std::string>(readFile(narPath, true))));
+        list(makeNarAccessor(make_ref<std::string>(readFile(narPath))));
     }
 };