Re: [PATCH] Btrfs: add a disk info ioctl to get the disks attached to a filesystem

[Josef Bacik <josef@redhat.com>]

On Wed, Sep 29, 2010 at 07:43:27PM -0400, Christoph Hellwig wrote:
> On Wed, Sep 29, 2010 at 10:04:31AM +0200, Kay Sievers wrote:
> > On Wed, Sep 29, 2010 at 09:25, Ric Wheeler <rwheeler@redhat.com> wrote:
> > 
> > > Second question is why is checking in /sys a big deal, would ??you prefer an
> > > interface like we did for alignment in libblkid?
> > 
> > It's about knowing what's behind the 'nodev' major == 0 of a btrfs
> > mount. There is no way to get that from /sys or anywhere else at the
> > moment.
> > 
> > Usually filesystems backed by a disk have the dev_t of the device, or
> > the fake block devices like md/dm/raid have their own major and the
> > slaves/ directory pointing to the devices.
> > 
> > This is not only about readahead, it's every other tool, that needs to
> > know what kind of disks are behind a btrfs 'nodev' major == 0 mount.
> 
> Thanks for explaining the problem.  It's one that affects everything
> with more than one underlying block device, so adding a
> filesystem-specific ioctl hack is not a good idea.  As mentioned in this
> mail we already have a solution for that - the block device slaves
> links used for raid and volume managers.  The most logical fix is to
> re-use that for btrfs as well and stop it from abusing the anonymous
> block major that was never intended for block based filesystems (and
> already has caused trouble in other areas).  One way to to this might
> be to allocate a block major for btrfs that only gets used for
> representing these links.
>

Fair enough, I will look into this next week sometime.  Thanks,

Josef