On Mon, May 17, 2021 at 10:21:33PM -0700, Motohiro Kawahito wrote: What is the host file? Raw? qcow2? The whole architecture is not designed for performance in this sort of case. Essentially it is single threaded and making a round trip on every 512 byte request. See: https://libguestfs.org/guestfs-internals.1.html#architecture Yes, that should be fine, although I don't think it will make your case faster. Possibly. Do you know where it is being called from 3 times? Stepping back, I would say that this is the wrong way to go about this (unless you just want to benchmark libguestfs itself, which will only tell you that it's slow). If the host file is raw, you can just read it. If the host file is qcow2, try something like this instead, it will be far faster: $ nbdsh \ -c 'h.connect_systemd_socket_activation(["qemu-nbd", "-r", "-f", "qcow2", "test.qcow2"])' \ -c 'print ("%r" % h.pread(512,0))' You could also write it in C: https://rwmj.wordpress.com/2019/10/03/how-to-edit-a-qcow2-file-from-c/ Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html