ProFTPD module mod_fsync

 The mod_fsync module attempts to prevent such bottlenecks by forcibly
 flushing to disk the buffers used for files open for writing after a
 certain number of bytes have been written (for example, after 128 KB
 has been written to a file). This prevents the buffer cache from being
 dominated by data from files being written, freeing up space for data
 for files being read.