diff --git a/io/fs/disk.go b/io/fs/disk.go
index fc614556..ee6b3ff4 100644
--- a/io/fs/disk.go
+++ b/io/fs/disk.go
@@ -203,7 +203,7 @@ func NewRootedDiskFilesystem(config RootedDiskConfig) (Filesystem, error) {
 
 	info, err := os.Stat(fs.root)
 	if err != nil {
-		return nil, os.ErrNotExist
+		return nil, ErrNotExist
 	}
 
 	if !info.IsDir() {
@@ -285,7 +285,7 @@ func (fs *diskFilesystem) Symlink(oldname, newname string) error {
 
 	info, err := os.Lstat(oldname)
 	if err != nil {
-		return os.ErrNotExist
+		return ErrNotExist
 	}
 
 	if info.Mode()&os.ModeSymlink != 0 {
@@ -494,7 +494,7 @@ func (fs *diskFilesystem) Stat(path string) (FileInfo, error) {
 
 	info, err := os.Lstat(path)
 	if err != nil {
-		return nil, os.ErrNotExist
+		return nil, ErrNotExist
 	}
 
 	dif.mode = info.Mode()
@@ -502,7 +502,7 @@ func (fs *diskFilesystem) Stat(path string) (FileInfo, error) {
 	if info.Mode()&os.ModeSymlink != 0 {
 		info, err = os.Stat(path)
 		if err != nil {
-			return nil, os.ErrNotExist
+			return nil, ErrNotExist
 		}
 	}
 
@@ -674,7 +674,7 @@ func (fs *diskFilesystem) LookPath(file string) (string, error) {
 		if err == nil {
 			return file, nil
 		}
-		return "", os.ErrNotExist
+		return "", ErrNotExist
 	}
 	path := os.Getenv("PATH")
 	for _, dir := range filepath.SplitList(path) {
@@ -686,12 +686,12 @@ func (fs *diskFilesystem) LookPath(file string) (string, error) {
 		path = fs.cleanPath(path)
 		if err := fs.findExecutable(path); err == nil {
 			if !filepath.IsAbs(path) {
-				return path, os.ErrNotExist
+				return path, ErrNotExist
 			}
 			return path, nil
 		}
 	}
-	return "", os.ErrNotExist
+	return "", ErrNotExist
 }
 
 func (fs *diskFilesystem) findExecutable(file string) error {
diff --git a/io/fs/fs.go b/io/fs/fs.go
index 72f36c53..3d162533 100644
--- a/io/fs/fs.go
+++ b/io/fs/fs.go
@@ -2,12 +2,16 @@
 package fs
 
 import (
+	"errors"
 	"io"
 	"io/fs"
 	"os"
 	"time"
 )
 
+var ErrExist = errors.New("file or directory already exists")
+var ErrNotExist = errors.New("file or directory does not exist")
+
 // FileInfo describes a file and is returned by Stat.
 type FileInfo interface {
 	// Name returns the full name of the file.
@@ -67,9 +71,9 @@ type ReadFilesystem interface {
 	// the number of bytes read or an error.
 	ReadFile(path string) ([]byte, error)
 
-	// Stat returns info about the file at path. If the file doesn't exist, an error
-	// will be returned. If the file is a symlink, the info reports the name and mode
-	// of the link itself, but the modification time and size are of the linked file.
+	// Stat returns info about the file at path. If the file doesn't exist, the error
+	// ErrNotExist will be returned. If the file is a symlink, the info reports the name
+	// and mode of the link itself, but the modification time and size are of the linked file.
 	Stat(path string) (FileInfo, error)
 
 	// List lists all files that are currently on the filesystem.
@@ -78,12 +82,13 @@ type ReadFilesystem interface {
 	// LookPath searches for an executable named file in the directories named by the PATH environment
 	// variable. If file contains a slash, it is tried directly and the PATH is not consulted. Otherwise,
 	// on success, the result is an absolute path. On non-disk filesystems. Only the mere existence
-	// of that file is verfied.
+	// of that file is verfied. In case the file is not found, the error ErrNotExist will be returned.
 	LookPath(file string) (string, error)
 }
 
 type WriteFilesystem interface {
-	// Symlink creates newname as a symbolic link to oldname.
+	// Symlink creates newname as a symbolic link to oldname. Return ErrNotExist if oldname doesn't exist.
+	// If newname already exists, ErrExits will be returned.
 	Symlink(oldname, newname string) error
 
 	// WriteFileReader adds a file to the filesystem. Returns the size of the data that has been
@@ -105,6 +110,7 @@ type WriteFilesystem interface {
 	// MkdirAll creates a directory named path, along with any necessary parents, and returns nil,
 	// or else returns an error. The permission bits perm (before umask) are used for all directories
 	// that MkdirAll creates. If path is already a directory, MkdirAll does nothing and returns nil.
+	// If the path already exists and is a regular file, ErrExists will be returned.
 	MkdirAll(path string, perm os.FileMode) error
 
 	// Rename renames the file from src to dst. If src and dst can't be renamed
diff --git a/io/fs/mem.go b/io/fs/mem.go
index baaee301..9a055022 100644
--- a/io/fs/mem.go
+++ b/io/fs/mem.go
@@ -289,13 +289,13 @@ func (fs *memFilesystem) ReadFile(path string) ([]byte, error) {
 	fs.filesLock.RUnlock()
 
 	if !ok {
-		return nil, os.ErrNotExist
+		return nil, ErrNotExist
 	}
 
 	if len(file.linkTo) != 0 {
 		file, ok = fs.files[file.linkTo]
 		if !ok {
-			return nil, os.ErrNotExist
+			return nil, ErrNotExist
 		}
 	}
 
@@ -310,11 +310,11 @@ func (fs *memFilesystem) Symlink(oldname, newname string) error {
 	defer fs.filesLock.Unlock()
 
 	if _, ok := fs.files[oldname]; !ok {
-		return os.ErrNotExist
+		return ErrNotExist
 	}
 
 	if _, ok := fs.files[newname]; ok {
-		return os.ErrExist
+		return ErrExist
 	}
 
 	if file, ok := fs.files[oldname]; ok {
@@ -465,7 +465,7 @@ func (fs *memFilesystem) MkdirAll(path string, perm os.FileMode) error {
 			return nil
 		}
 
-		return os.ErrExist
+		return ErrExist
 	}
 
 	f := &internalMemFile{
@@ -495,7 +495,7 @@ func (fs *memFilesystem) Rename(src, dst string) error {
 
 	srcFile, ok := fs.files[src]
 	if !ok {
-		return os.ErrNotExist
+		return ErrNotExist
 	}
 
 	dstFile, ok := fs.files[dst]
@@ -524,11 +524,11 @@ func (fs *memFilesystem) Copy(src, dst string) error {
 
 	srcFile, ok := fs.files[src]
 	if !ok {
-		return os.ErrNotExist
+		return ErrNotExist
 	}
 
 	if srcFile.dir {
-		return os.ErrNotExist
+		return ErrNotExist
 	}
 
 	if fs.isDir(dst) {
@@ -582,7 +582,7 @@ func (fs *memFilesystem) stat(path string) (FileInfo, error) {
 		if len(f.linkTo) != 0 {
 			file, ok := fs.files[f.linkTo]
 			if !ok {
-				return nil, os.ErrNotExist
+				return nil, ErrNotExist
 			}
 
 			f.lastMod = file.lastMod
@@ -594,7 +594,7 @@ func (fs *memFilesystem) stat(path string) (FileInfo, error) {
 
 	// Check for directories
 	if !fs.isDir(path) {
-		return nil, os.ErrNotExist
+		return nil, ErrNotExist
 	}
 
 	f := &memFileInfo{
@@ -799,11 +799,11 @@ func (fs *memFilesystem) LookPath(file string) (string, error) {
 		info, err := fs.Stat(file)
 		if err == nil {
 			if !info.Mode().IsRegular() {
-				return file, os.ErrNotExist
+				return file, ErrNotExist
 			}
 			return file, nil
 		}
-		return "", os.ErrNotExist
+		return "", ErrNotExist
 	}
 	path := os.Getenv("PATH")
 	for _, dir := range filepath.SplitList(path) {
@@ -815,15 +815,15 @@ func (fs *memFilesystem) LookPath(file string) (string, error) {
 		path = fs.cleanPath(path)
 		if info, err := fs.Stat(path); err == nil {
 			if !filepath.IsAbs(path) {
-				return path, os.ErrNotExist
+				return path, ErrNotExist
 			}
 			if !info.Mode().IsRegular() {
-				return path, os.ErrNotExist
+				return path, ErrNotExist
 			}
 			return path, nil
 		}
 	}
-	return "", os.ErrNotExist
+	return "", ErrNotExist
 }
 
 func (fs *memFilesystem) cleanPath(path string) string {
diff --git a/io/fs/s3.go b/io/fs/s3.go
index 78fc1411..6d7807d8 100644
--- a/io/fs/s3.go
+++ b/io/fs/s3.go
@@ -270,7 +270,7 @@ func (fs *s3Filesystem) ReadFile(path string) ([]byte, error) {
 	path = fs.cleanPath(path)
 	file := fs.Open(path)
 	if file == nil {
-		return nil, os.ErrNotExist
+		return nil, ErrNotExist
 	}
 
 	defer file.Close()
@@ -403,7 +403,7 @@ func (fs *s3Filesystem) MkdirAll(path string, perm os.FileMode) error {
 	info, err := fs.Stat(path)
 	if err == nil {
 		if !info.IsDir() {
-			return os.ErrExist
+			return ErrExist
 		}
 
 		return nil
@@ -624,11 +624,11 @@ func (fs *s3Filesystem) LookPath(file string) (string, error) {
 		info, err := fs.Stat(file)
 		if err == nil {
 			if !info.Mode().IsRegular() {
-				return file, os.ErrNotExist
+				return file, ErrNotExist
 			}
 			return file, nil
 		}
-		return "", os.ErrNotExist
+		return "", ErrNotExist
 	}
 	path := os.Getenv("PATH")
 	for _, dir := range filepath.SplitList(path) {
@@ -640,15 +640,15 @@ func (fs *s3Filesystem) LookPath(file string) (string, error) {
 		path = fs.cleanPath(path)
 		if info, err := fs.Stat(path); err == nil {
 			if !filepath.IsAbs(path) {
-				return path, os.ErrNotExist
+				return path, ErrNotExist
 			}
 			if !info.Mode().IsRegular() {
-				return path, os.ErrNotExist
+				return path, ErrNotExist
 			}
 			return path, nil
 		}
 	}
-	return "", os.ErrNotExist
+	return "", ErrNotExist
 }
 
 func (fs *s3Filesystem) isDir(path string) bool {
diff --git a/session/snapshot.go b/session/snapshot.go
index 82c61d1e..f8d6ea51 100644
--- a/session/snapshot.go
+++ b/session/snapshot.go
@@ -2,9 +2,7 @@ package session
 
 import (
 	"bytes"
-	"errors"
 	"io"
-	"os"
 
 	"github.com/datarhei/core/v16/io/fs"
 )
@@ -34,14 +32,14 @@ type historySource struct {
 // session history. If there's no data, a nil source with a nil error will be returned.
 // If there's data, a non-nil source with a nil error will be returned. Otherwise
 // the source will be nil and the error non-nil.
-func NewHistorySource(fs fs.Filesystem, path string) (SnapshotSource, error) {
+func NewHistorySource(filesystem fs.Filesystem, path string) (SnapshotSource, error) {
 	s := &historySource{
-		fs:   fs,
+		fs:   filesystem,
 		path: path,
 	}
 
 	if _, err := s.fs.Stat(s.path); err != nil {
-		if errors.Is(err, os.ErrNotExist) {
+		if err == fs.ErrNotExist {
 			return nil, nil
 		}
 
diff --git a/session/snapshot_test.go b/session/snapshot_test.go
index b98cf26e..10f9918b 100644
--- a/session/snapshot_test.go
+++ b/session/snapshot_test.go
@@ -11,7 +11,7 @@ func TestHistorySource(t *testing.T) {
 	memfs, err := fs.NewMemFilesystem(fs.MemConfig{})
 	require.NoError(t, err)
 
-	s, err := NewHistorySource(memfs, "/foobar.json")
+	s, err := NewHistorySource(memfs, "./foobar.json")
 	require.NoError(t, err)
 	require.Nil(t, s)
 }