vertx.fileSystem().open(testDir + pathSep + fileName, new OpenOptions(), ar -> {
if (ar.succeeded()) {
AsyncFile ws = ar.result();
long size = vertx.fileSystem().propsSync(testDir + pathSep + fileName).size();
ws.setWritePos(size);
ws.exceptionHandler(t -> fail(t.getMessage()));
for (int i = 0; i < chunks; i++) {
Buffer chunk = buff.getBuffer(i * chunkSize, (i + 1) * chunkSize);
assertEquals(chunkSize, chunk.length());
ws.write(chunk);
}