From: "Richard W.M. Jones" <rjo...@redhat.com> 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 <rjo...@redhat.com> BZ: https://bugzilla.redhat.com/show_bug.cgi?id=1265196 Reviewed-by: Jeff Cody <jc...@redhat.com> Reviewed-by: Eric Blake <ebl...@redhat.com> Signed-off-by: Kevin Wolf <kw...@redhat.com> --- block/raw-posix.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/block/raw-posix.c b/block/raw-posix.c index 30df8ad..86f8562 100644 --- a/block/raw-posix.c +++ b/block/raw-posix.c @@ -1648,7 +1648,7 @@ static int raw_create(const char *filename, QemuOpts *opts, Error **errp) goto out; } - fd = qemu_open(filename, O_WRONLY | O_CREAT | O_TRUNC | O_BINARY, + fd = qemu_open(filename, O_RDWR | O_CREAT | O_TRUNC | O_BINARY, 0644); if (fd < 0) { result = -errno; -- 1.8.3.1
