Re: [Qemu-devel] [PATCH] qemu-img: set nocow flag to new file

[Alex Bligh]

--On 14 November 2013 10:17:26 +0100 Kevin Wolf <address@hidden> wrote:

> > +#ifdef __linux__
> > +        /* set NOCOW flag to solve performance issue on fs like btrfs */
> > +        int attr;
> > +        attr = FS_NOCOW_FL;
> > +        ioctl(fd, FS_IOC_SETFLAGS, &attr);
> > +#endif
>
> ioctl() returning an error is ignored. This is probably okay because
> we're only talking about an optimisation here. Perhaps worth a word or
> two in the comment.
>
> However, while this ioctl is setting FS_NOCOW_FL, it is at the same time
> clearing all other flags. This doesn't look right.

Also, given FS_NOCOW_FL was only introduced in 2.6.39, should this not
be guarded by
#ifdef FS_NOCOW_FL
(or better tested in configure in case it becomes something other than
a #define in which case this test could replace #ifdef __linux__)

--
Alex Bligh