diff --git a/module_test.go b/module_test.go
index 9587dbe..b6eb28a 100644
--- a/module_test.go
+++ b/module_test.go
@@ -76,7 +76,6 @@ func TestConn_newArena(t *testing.T) {
 	defer arena.free()
 
 	const title = "Lorem ipsum"
-
 	ptr := arena.string(title)
 	if ptr == 0 {
 		t.Fatalf("got nullptr")
@@ -93,6 +92,19 @@ func TestConn_newArena(t *testing.T) {
 	if got := util.ReadString(m.mod, ptr, math.MaxUint32); got != body {
 		t.Errorf("got %q, want %q", got, body)
 	}
+
+	ptr = arena.bytes(nil)
+	if ptr != 0 {
+		t.Errorf("want nullptr")
+	}
+	ptr = arena.bytes([]byte(title))
+	if ptr == 0 {
+		t.Fatalf("got nullptr")
+	}
+	if got := util.View(m.mod, ptr, uint64(len(title))); string(got) != title {
+		t.Errorf("got %q, want %q", got, title)
+	}
+
 	arena.free()
 }
 
diff --git a/sqlite3vfs/memory.go b/sqlite3vfs/memory.go
index b6e114c..fcc50b6 100644
--- a/sqlite3vfs/memory.go
+++ b/sqlite3vfs/memory.go
@@ -43,6 +43,9 @@ func (vfs MemoryVFS) FullPathname(name string) (string, error) {
 
 const memSectorSize = 65536
 
+// A MemoryDB is a [MemoryVFS] database.
+//
+// A MemoryDB is safe to access concurrently from multiple SQLite connections.
 type MemoryDB struct {
 	mtx  sync.RWMutex
 	size int64
@@ -60,10 +63,12 @@ type memoryFile struct {
 	readOnly bool
 }
 
+// Close implements the [File] and [io.Closer] interfaces.
 func (m *memoryFile) Close() error {
 	return m.Unlock(LOCK_NONE)
 }
 
+// ReadAt implements the [File] and [io.ReaderAt] interfaces.
 func (m *memoryFile) ReadAt(b []byte, off int64) (n int, err error) {
 	m.mtx.RLock()
 	defer m.mtx.RUnlock()
@@ -71,31 +76,40 @@ func (m *memoryFile) ReadAt(b []byte, off int64) (n int, err error) {
 	if off >= m.size {
 		return 0, io.EOF
 	}
+
 	base := off / memSectorSize
 	rest := off % memSectorSize
 	have := int64(memSectorSize)
 	if base == int64(len(m.data))-1 {
-		have = m.size % memSectorSize
+		have = modRoundUp(m.size, memSectorSize)
 	}
-	return copy(b, (*m.data[base])[rest:have]), nil
+	n = copy(b, (*m.data[base])[rest:have])
+	if n < len(b) {
+		// Assume reads are page aligned.
+		return 0, io.ErrNoProgress
+	}
+	return n, nil
 }
 
+// WriteAt implements the [File] and [io.WriterAt] interfaces.
 func (m *memoryFile) WriteAt(b []byte, off int64) (n int, err error) {
 	m.mtx.Lock()
 	defer m.mtx.Unlock()
 
 	base := off / memSectorSize
 	rest := off % memSectorSize
-	if base >= int64(len(m.data)) {
+	for base >= int64(len(m.data)) {
 		m.data = append(m.data, new([memSectorSize]byte))
 	}
 	n = copy((*m.data[base])[rest:], b)
-	if size := off + int64(n); size > m.size {
-		m.size = size
+	if n < len(b) {
+		// Assume writes are page aligned.
+		return 0, io.ErrShortWrite
 	}
 	return n, nil
 }
 
+// Truncate implements the [File] interface.
 func (m *memoryFile) Truncate(size int64) error {
 	m.mtx.Lock()
 	defer m.mtx.Unlock()
@@ -106,31 +120,33 @@ func (m *memoryFile) truncate(size int64) error {
 	if size < m.size {
 		base := size / memSectorSize
 		rest := size % memSectorSize
-		clear((*m.data[base])[rest:])
+		if rest != 0 {
+			clear((*m.data[base])[rest:])
+		}
 	}
-	sectors := (size + memSectorSize - 1) / memSectorSize
+	sectors := divRoundUp(size, memSectorSize)
 	for sectors > int64(len(m.data)) {
 		m.data = append(m.data, new([memSectorSize]byte))
 	}
-	for sectors < int64(len(m.data)) {
-		last := int64(len(m.data)) - 1
-		m.data[last] = nil
-		m.data = m.data[:last]
-	}
+	clear(m.data[sectors:])
+	m.data = m.data[:sectors]
 	m.size = size
 	return nil
 }
 
+// Sync implements the [File] interface.
 func (*memoryFile) Sync(flag SyncFlag) error {
 	return nil
 }
 
+// Size implements the [File] interface.
 func (m *memoryFile) Size() (int64, error) {
 	m.mtx.RLock()
 	defer m.mtx.RUnlock()
 	return m.size, nil
 }
 
+// Lock implements the [File] interface.
 func (m *memoryFile) Lock(lock LockLevel) error {
 	if m.lock >= lock {
 		return nil
@@ -185,6 +201,7 @@ func (m *memoryFile) Lock(lock LockLevel) error {
 	return nil
 }
 
+// Unlock implements the [File] interface.
 func (m *memoryFile) Unlock(lock LockLevel) error {
 	if m.lock <= lock {
 		return nil
@@ -206,6 +223,7 @@ func (m *memoryFile) Unlock(lock LockLevel) error {
 	return nil
 }
 
+// CheckReservedLock implements the [File] interface.
 func (m *memoryFile) CheckReservedLock() (bool, error) {
 	if m.lock >= LOCK_RESERVED {
 		return true, nil
@@ -215,10 +233,12 @@ func (m *memoryFile) CheckReservedLock() (bool, error) {
 	return m.reserved != nil, nil
 }
 
+// SectorSize implements the [File] interface.
 func (*memoryFile) SectorSize() int {
 	return memSectorSize
 }
 
+// DeviceCharacteristics implements the [File] interface.
 func (*memoryFile) DeviceCharacteristics() DeviceCharacteristic {
 	return IOCAP_ATOMIC |
 		IOCAP_SEQUENTIAL |
@@ -226,6 +246,7 @@ func (*memoryFile) DeviceCharacteristics() DeviceCharacteristic {
 		IOCAP_POWERSAFE_OVERWRITE
 }
 
+// SizeHint implements the [FileSizeHint] interface.
 func (m *memoryFile) SizeHint(size int64) error {
 	m.mtx.Lock()
 	defer m.mtx.Unlock()
@@ -235,12 +256,22 @@ func (m *memoryFile) SizeHint(size int64) error {
 	return nil
 }
 
+// LockState implements the [FileLockState] interface.
 func (m *memoryFile) LockState() LockLevel {
 	return m.lock
 }
 
-func clear(b []byte) {
+func divRoundUp(a, b int64) int64 {
+	return (a + b - 1) / b
+}
+
+func modRoundUp(a, b int64) int64 {
+	return b - (b-a%b)%b
+}
+
+func clear[T any](b []T) {
+	var zero T
 	for i := range b {
-		b[i] = 0
+		b[i] = zero
 	}
 }
diff --git a/sqlite3vfs/reader_test.go b/sqlite3vfs/reader_test.go
index 192d6aa..34e856f 100644
--- a/sqlite3vfs/reader_test.go
+++ b/sqlite3vfs/reader_test.go
@@ -3,6 +3,7 @@ package sqlite3vfs_test
 import (
 	"database/sql"
 	"fmt"
+	"io"
 	"log"
 	"os"
 	"path/filepath"
@@ -125,7 +126,23 @@ func TestReaderVFS_Open(t *testing.T) {
 }
 
 func TestNewSizeReaderAt(t *testing.T) {
-	n, err := sqlite3vfs.NewSizeReaderAt(strings.NewReader("abc")).Size()
+	f, err := os.Create(filepath.Join(t.TempDir(), "abc.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+
+	n, err := sqlite3vfs.NewSizeReaderAt(f).Size()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 0 {
+		t.Errorf("got %d", n)
+	}
+
+	reader := strings.NewReader("abc")
+
+	n, err = sqlite3vfs.NewSizeReaderAt(reader).Size()
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -133,17 +150,55 @@ func TestNewSizeReaderAt(t *testing.T) {
 		t.Errorf("got %d", n)
 	}
 
-	f, err := os.Create(filepath.Join(t.TempDir(), "abc.txt"))
+	n, err = sqlite3vfs.NewSizeReaderAt(lener{reader, reader.Len()}).Size()
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer f.Close()
-
-	n, err = sqlite3vfs.NewSizeReaderAt(f).Size()
-	if err != nil {
-		t.Fatal(err)
-	}
-	if n != 0 {
+	if n != 3 {
 		t.Errorf("got %d", n)
 	}
+
+	n, err = sqlite3vfs.NewSizeReaderAt(sizer{reader, reader.Size()}).Size()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 3 {
+		t.Errorf("got %d", n)
+	}
+
+	n, err = sqlite3vfs.NewSizeReaderAt(seeker{reader, reader}).Size()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if n != 3 {
+		t.Errorf("got %d", n)
+	}
+
+	_, err = sqlite3vfs.NewSizeReaderAt(readerat{reader}).Size()
+	if err == nil {
+		t.Error("want error")
+	}
+}
+
+type lener struct {
+	io.ReaderAt
+	len int
+}
+
+func (l lener) Len() int { return l.len }
+
+type sizer struct {
+	io.ReaderAt
+	size int64
+}
+
+func (l sizer) Size() (int64, error) { return l.size, nil }
+
+type seeker struct {
+	io.ReaderAt
+	io.Seeker
+}
+
+type readerat struct {
+	io.ReaderAt
 }
diff --git a/sqlite3vfs/tests/mptest/mptest_test.go b/sqlite3vfs/tests/mptest/mptest_test.go
index 1dd088d..59b8298 100644
--- a/sqlite3vfs/tests/mptest/mptest_test.go
+++ b/sqlite3vfs/tests/mptest/mptest_test.go
@@ -32,6 +32,7 @@ var (
 	rt        wazero.Runtime
 	module    wazero.CompiledModule
 	instances atomic.Uint64
+	memory    = sqlite3vfs.MemoryVFS{}
 )
 
 func init() {
@@ -51,6 +52,8 @@ func init() {
 	if err != nil {
 		panic(err)
 	}
+
+	sqlite3vfs.Register("memvfs", memory)
 }
 
 func config(ctx context.Context) wazero.ModuleConfig {
@@ -72,11 +75,28 @@ func system(ctx context.Context, mod api.Module, ptr uint32) uint32 {
 	buf, _ := mod.Memory().Read(ptr, mod.Memory().Size()-ptr)
 	buf = buf[:bytes.IndexByte(buf, 0)]
 
+	var memvfs, journal, timeout bool
 	args := strings.Split(string(buf), " ")
 	for i := range args {
 		args[i] = strings.Trim(args[i], `"`)
+		switch args[i] {
+		case "memvfs":
+			memvfs = true
+		case "--timeout":
+			timeout = true
+		case "--journalmode":
+			journal = true
+		}
 	}
 	args = args[:len(args)-1]
+	if memvfs {
+		if !timeout {
+			args = append(args, "--timeout", "1000")
+		}
+		if !journal {
+			args = append(args, "--journalmode", "memory")
+		}
+	}
 
 	cfg := config(ctx).WithArgs(args...)
 	go func() {
@@ -151,6 +171,42 @@ func Test_multiwrite01(t *testing.T) {
 	vfs.Close()
 }
 
+func Test_config01_memory(t *testing.T) {
+	memory["test.db"] = new(sqlite3vfs.MemoryDB)
+	ctx, vfs := sqlite3vfs.NewContext(newContext(t))
+	cfg := config(ctx).WithArgs("mptest", "test.db",
+		"config01.test",
+		"--vfs", "memvfs",
+		"--timeout", "1000",
+		"--journalmode", "memory")
+	mod, err := rt.InstantiateModule(ctx, module, cfg)
+	if err != nil {
+		t.Error(err)
+	}
+	mod.Close(ctx)
+	vfs.Close()
+}
+
+func Test_multiwrite01_memory(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+
+	memory["test.db"] = new(sqlite3vfs.MemoryDB)
+	ctx, vfs := sqlite3vfs.NewContext(newContext(t))
+	cfg := config(ctx).WithArgs("mptest", "test.db",
+		"multiwrite01.test",
+		"--vfs", "memvfs",
+		"--timeout", "1000",
+		"--journalmode", "memory")
+	mod, err := rt.InstantiateModule(ctx, module, cfg)
+	if err != nil {
+		t.Error(err)
+	}
+	mod.Close(ctx)
+	vfs.Close()
+}
+
 func newContext(t *testing.T) context.Context {
 	return context.WithValue(context.Background(), logger{}, &testWriter{T: t})
 }