private void removeOld() throws FileStorageException {
        final FileSystemService fileSystemService = new FileSystemService();
        final Properties expirationDates = systemInformation.getExpirationDates();
        final Set<String> keys = expirationDates.stringPropertyNames();
        for (String key : keys) {
            final String expirationDate = expirationDates.getProperty(key);
            if(expirationDate == null){
                continue;
            }
            final long time = Long.parseLong(expirationDate);
            if (expirationDate != null && time != Item.WITHOUT_EXPIRATION) {
                Item item = systemInformation.get(key);
                if (item == null) {
                    continue;
                }
                if ((item.getExpirationTime() + item.getCreationTime()) < System.currentTimeMillis()) {
                    fileSystemService.removeFile(systemInformation.getRootPath() + item.getPath());
                    systemInformation.remove(item);
                }
            }
        }
    }