On 09/29/2015 09:54 AM, Richard W.M. Jones wrote: > https://bugzilla.redhat.com/show_bug.cgi?id=1265196 > > The following command fails on an NFS mountpoint: > > $ qemu-img create -f qcow2 -o preallocation=falloc disk.img 262144 > Formatting 'disk.img', fmt=qcow2 size=262144 encryption=off > cluster_size=65536 preallocation='falloc' lazy_refcounts=off > qemu-img: disk.img: Could not preallocate data for the new file: Bad file > descriptor > > The reason turns out to be because NFS doesn't support the > posix_fallocate call. glibc emulates it instead. However glibc's > emulation involves using the pread(2) syscall. The pread syscall > fails with EBADF if the file descriptor is opened without the read > open-flag (ie. open (..., O_WRONLY)). >
Failing with EBADF feels fishy (POSIX requests EINVAL in that case) - but that's an upstream glibc discussion that has forked from this thread and doesn't hold up the qemu fix. > I contacted glibc upstream about this, and their response is here: > > https://bugzilla.redhat.com/show_bug.cgi?id=1265196#c9 > > There are two possible fixes: Use Linux fallocate directly, or (this > fix) work around the problem in qemu by opening the file with O_RDWR > instead of O_WRONLY. > > Signed-off-by: Richard W.M. Jones <rjo...@redhat.com> > BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196 > --- > block/raw-posix.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) Reviewed-by: Eric Blake <ebl...@redhat.com> -- Eric Blake eblake redhat com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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