This isn't a problem with qemu as such, but it may affect you if you use qemu with a newer glibc on a Linux kernel which does not support the preadv(2) syscall natively. glibc will attempt to emulate preadv(2) using pread(2) syscalls, but it doesn't align the user buffer when it does this, and that can break things (if using cache=off then the file is opened with O_DIRECT which has strict alignment requirements).
The user visible effect of this bug is that blocks devices (of all types) inside the guest throw large numbers of I/O errors. https://bugzilla.redhat.com/show_bug.cgi?id=563103#c8 One interesting thing is that qemu has its own preadv emulation (which does the emulation correctly), but this is never used because qemu never gets ENOSYS back from preadv. Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming blog: http://rwmj.wordpress.com Fedora now supports 80 OCaml packages (the OPEN alternative to F#) http://cocan.org/getting_started_with_ocaml_on_red_hat_and_fedora
