Re: [Qemu-devel] [PATCH] block/raw-posix: Open file descriptor O_RDWR to work around glibc posix_fallocate emulation issue.

[Kevin Wolf <kwolf@redhat.com>]

Am 29.09.2015 um 17:54 hat Richard W.M. Jones geschrieben:
>   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

Thanks, applied to the block branch.

Kevin