diff --git a/io/fs/disk.go b/io/fs/disk.go
index 6cac9d40..b9f76b77 100644
--- a/io/fs/disk.go
+++ b/io/fs/disk.go
@@ -403,6 +403,31 @@ func (fs *diskFilesystem) WriteFileSafe(path string, data []byte) (int64, bool,
 	return int64(size), !replace, nil
 }
 
+func (fs *diskFilesystem) AppendFileReader(path string, r io.Reader, sizeHint int) (int64, error) {
+	path = fs.cleanPath(path)
+
+	dir := filepath.Dir(path)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return -1, fmt.Errorf("creating file failed: %w", err)
+	}
+
+	f, err := os.OpenFile(path, os.O_APPEND|os.O_WRONLY|os.O_CREATE, 0644)
+	if err != nil {
+		return -1, err
+	}
+
+	defer f.Close()
+
+	size, err := f.ReadFrom(r)
+	if err != nil {
+		return -1, fmt.Errorf("reading data failed: %w", err)
+	}
+
+	fs.lastSizeCheck = time.Time{}
+
+	return size, nil
+}
+
 func (fs *diskFilesystem) Rename(src, dst string) error {
 	src = fs.cleanPath(src)
 	dst = fs.cleanPath(dst)
diff --git a/io/fs/fs.go b/io/fs/fs.go
index 4cc8e201..973be194 100644
--- a/io/fs/fs.go
+++ b/io/fs/fs.go
@@ -108,6 +108,10 @@ type WriteFilesystem interface {
 	// an error adding the file and error is not nil.
 	WriteFileSafe(path string, data []byte) (int64, bool, error)
 
+	// AppendFileReader appends the contents from reader to the file at path. If the file doesn't
+	// exist, it will be created. The number of written bytes will be returned, -1 otherwise.
+	AppendFileReader(path string, r io.Reader, size int) (int64, error)
+
 	// 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.
diff --git a/io/fs/fs_test.go b/io/fs/fs_test.go
index 7e7c2362..17d79e3c 100644
--- a/io/fs/fs_test.go
+++ b/io/fs/fs_test.go
@@ -115,6 +115,8 @@ func TestFilesystem(t *testing.T) {
 		"symlinkErrors":   testSymlinkErrors,
 		"symlinkOpenStat": testSymlinkOpenStat,
 		"open":            testOpen,
+		"append":          testAppend,
+		"appendCreate":    testAppendCreate,
 	}
 
 	for fsname, fs := range filesystems {
@@ -125,6 +127,11 @@ func TestFilesystem(t *testing.T) {
 				}
 				filesystem, err := fs(name)
 				require.NoError(t, err)
+
+				if fsname == "s3fs" {
+					filesystem.RemoveList("/", ListOptions{Pattern: "/**"})
+				}
+
 				test(t, filesystem)
 			})
 		}
@@ -859,3 +866,28 @@ func testSymlinkErrors(t *testing.T, fs Filesystem) {
 	err = fs.Symlink("/bazfoo", "/barfoo")
 	require.Error(t, err)
 }
+
+func testAppend(t *testing.T, fs Filesystem) {
+	_, _, err := fs.WriteFileReader("/foobar", strings.NewReader("part1"), -1)
+	require.NoError(t, err)
+
+	_, err = fs.AppendFileReader("/foobar", strings.NewReader("part2"), -1)
+	require.NoError(t, err)
+
+	file := fs.Open("/foobar")
+	require.NotNil(t, file)
+
+	data, err := io.ReadAll(file)
+	require.Equal(t, []byte("part1part2"), data)
+}
+
+func testAppendCreate(t *testing.T, fs Filesystem) {
+	_, err := fs.AppendFileReader("/foobar", strings.NewReader("part1"), -1)
+	require.NoError(t, err)
+
+	file := fs.Open("/foobar")
+	require.NotNil(t, file)
+
+	data, err := io.ReadAll(file)
+	require.Equal(t, []byte("part1"), data)
+}
diff --git a/io/fs/mem.go b/io/fs/mem.go
index dee83c78..de8b3cb7 100644
--- a/io/fs/mem.go
+++ b/io/fs/mem.go
@@ -525,6 +525,46 @@ func (fs *memFilesystem) WriteFileSafe(path string, data []byte) (int64, bool, e
 	return fs.WriteFileReader(path, bytes.NewReader(data), len(data))
 }
 
+func (fs *memFilesystem) AppendFileReader(path string, r io.Reader, sizeHint int) (int64, error) {
+	path = fs.cleanPath(path)
+
+	file, hasFile := fs.storage.LoadAndCopy(path)
+	if !hasFile {
+		size, _, err := fs.WriteFileReader(path, r, sizeHint)
+		return size, err
+	}
+
+	size, err := copyToBufferFromReader(file.data, r, 8*1024)
+	if err != nil {
+		fs.logger.WithFields(log.Fields{
+			"path":           path,
+			"filesize_bytes": size,
+			"error":          err,
+		}).Warn().Log("Incomplete file")
+
+		file.Close()
+
+		return -1, fmt.Errorf("incomplete file")
+	}
+
+	file.size += size
+
+	fs.storage.Store(path, file)
+
+	fs.sizeLock.Lock()
+	defer fs.sizeLock.Unlock()
+
+	fs.currentSize += size
+
+	fs.logger.Debug().WithFields(log.Fields{
+		"path":           file.name,
+		"filesize_bytes": file.size,
+		"size_bytes":     fs.currentSize,
+	}).Log("Appended to file")
+
+	return size, nil
+}
+
 func (fs *memFilesystem) Purge(size int64) int64 {
 	files := []*memFile{}
 
diff --git a/io/fs/mem_storage.go b/io/fs/mem_storage.go
index 5df8f946..044b375e 100644
--- a/io/fs/mem_storage.go
+++ b/io/fs/mem_storage.go
@@ -9,11 +9,27 @@ import (
 )
 
 type memStorage interface {
+	// Delete deletes a file from the storage.
 	Delete(key string) (*memFile, bool)
+
+	// Store stores a file to the storage. If there's already a file with
+	// the same key, that value will be returned and replaced with the
+	// new file.
 	Store(key string, value *memFile) (*memFile, bool)
+
+	// Load loads a file from the storage. This is a references to the file,
+	// i.e. all changes to the file will be reflected on the storage.
 	Load(key string) (value *memFile, ok bool)
+
+	// LoadAndCopy loads a file from the storage. The returned file is a copy
+	// and can be modified without modifying the file on the storage.
 	LoadAndCopy(key string) (value *memFile, ok bool)
+
+	// Has checks whether a file exists at path.
 	Has(key string) bool
+
+	// Range ranges over all files on the storage. The callback needs to return
+	// false in order to stop the iteration.
 	Range(f func(key string, value *memFile) bool)
 }
 
diff --git a/io/fs/s3.go b/io/fs/s3.go
index c75162ed..ed9d1ade 100644
--- a/io/fs/s3.go
+++ b/io/fs/s3.go
@@ -360,6 +360,25 @@ func (fs *s3Filesystem) WriteFileSafe(path string, data []byte) (int64, bool, er
 	return fs.WriteFileReader(path, bytes.NewReader(data), len(data))
 }
 
+func (fs *s3Filesystem) AppendFileReader(path string, r io.Reader, sizeHint int) (int64, error) {
+	path = fs.cleanPath(path)
+
+	ctx := context.Background()
+
+	object, err := fs.client.GetObject(ctx, fs.bucket, path, minio.GetObjectOptions{})
+	if err != nil {
+		size, _, err := fs.write(path, r)
+		return size, err
+	}
+
+	buffer := bytes.Buffer{}
+	buffer.ReadFrom(object)
+	buffer.ReadFrom(r)
+
+	size, _, err := fs.write(path, &buffer)
+	return size, err
+}
+
 func (fs *s3Filesystem) Rename(src, dst string) error {
 	src = fs.cleanPath(src)
 	dst = fs.cleanPath(dst)
diff --git a/io/fs/sized.go b/io/fs/sized.go
index aa6b552b..cfa159eb 100644
--- a/io/fs/sized.go
+++ b/io/fs/sized.go
@@ -135,34 +135,40 @@ func (r *sizedFilesystem) WriteFileSafe(path string, data []byte) (int64, bool,
 	return r.Filesystem.WriteFileSafe(path, data)
 }
 
+func (r *sizedFilesystem) AppendFileReader(path string, rd io.Reader, sizeHint int) (int64, error) {
+	currentSize, maxSize := r.Size()
+	if maxSize <= 0 {
+		return r.Filesystem.AppendFileReader(path, rd, sizeHint)
+	}
+
+	data := bytes.Buffer{}
+	size, err := copyToBufferFromReader(&data, rd, 8*1024)
+	if err != nil {
+		return -1, err
+	}
+
+	// Calculate the new size of the filesystem
+	newSize := currentSize + size
+
+	// If the the new size is larger than the allowed size, we have to free
+	// some space.
+	if newSize > maxSize {
+		if !r.purge {
+			return -1, fmt.Errorf("not enough space on device")
+		}
+
+		if r.Purge(size) < size {
+			return -1, fmt.Errorf("not enough space on device")
+		}
+	}
+
+	return r.Filesystem.AppendFileReader(path, &data, int(size))
+}
+
 func (r *sizedFilesystem) Purge(size int64) int64 {
 	if purger, ok := r.Filesystem.(PurgeFilesystem); ok {
 		return purger.Purge(size)
 	}
 
 	return 0
-	/*
-	   files := r.Filesystem.List("/", "")
-
-	   	sort.Slice(files, func(i, j int) bool {
-	   		return files[i].ModTime().Before(files[j].ModTime())
-	   	})
-
-	   var freed int64 = 0
-
-	   	for _, f := range files {
-	   		r.Filesystem.Remove(f.Name())
-	   		size -= f.Size()
-	   		freed += f.Size()
-	   		r.currentSize -= f.Size()
-
-	   		if size <= 0 {
-	   			break
-	   		}
-	   	}
-
-	   files = nil
-
-	   return freed
-	*/
 }
diff --git a/session/registry.go b/session/registry.go
index 0dd54af6..1a08b3f0 100644
--- a/session/registry.go
+++ b/session/registry.go
@@ -199,12 +199,6 @@ func (r *registry) sessionPersister(pattern *strftime.Strftime, bufferDuration t
 	buffer := &bytes.Buffer{}
 	path := pattern.FormatString(time.Now())
 
-	file := r.persist.fs.Open(path)
-	if file != nil {
-		buffer.ReadFrom(file)
-		file.Close()
-	}
-
 	enc := json.NewEncoder(buffer)
 
 	ticker := time.NewTicker(bufferDuration)
@@ -222,7 +216,7 @@ loop:
 			currentPath := pattern.FormatString(session.ClosedAt)
 			if currentPath != path && session.ClosedAt.After(splitTime) {
 				if buffer.Len() > 0 {
-					_, _, err := r.persist.fs.WriteFileSafe(path, buffer.Bytes())
+					_, err := r.persist.fs.AppendFileReader(path, buffer, -1)
 					if err != nil {
 						r.logger.Error().WithError(err).WithField("path", path).Log("")
 					}
@@ -239,7 +233,7 @@ loop:
 			enc.Encode(&session)
 		case t := <-ticker.C:
 			if buffer.Len() > 0 {
-				_, _, err := r.persist.fs.WriteFileSafe(path, buffer.Bytes())
+				_, err := r.persist.fs.AppendFileReader(path, buffer, -1)
 				if err != nil {
 					r.logger.Error().WithError(err).WithField("path", path).Log("")
 				} else {
@@ -260,7 +254,7 @@ loop:
 	}
 
 	if buffer.Len() > 0 {
-		_, _, err := r.persist.fs.WriteFileSafe(path, buffer.Bytes())
+		_, err := r.persist.fs.AppendFileReader(path, buffer, -1)
 		if err != nil {
 			r.logger.Error().WithError(err).WithField("path", path).Log("")
 		} else {