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)).
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 <rjones@redhat.com>
BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
73ba05d936