Re: Extending FIEMAP ioctl to report device id

[Dave Chinner <david@fromorbit.com>]

On Mon, Feb 11, 2019 at 01:52:25PM -0700, Andreas Dilger wrote:
> On Feb 11, 2019, at 8:23 AM, Matthew Wilcox <willy@infradead.org> wrote:
> > 
> > On Mon, Feb 11, 2019 at 10:43:06AM +0100, Carlos Maiolino wrote:
> >> - The general idea, is to provide a way for FIEMAP ioctls to return the device
> >>  id where each extent is physically located.
> > 
> > How does userspace get to use this information?  If I call fiemap() and
> > it tells me extent 1 is on device 0x12345678 and extent 2 is on device
> > 0x34567812, what can I do with that information?
> 
> For filesystems that may store a file on different devices, filefrag will
> print out which device the file is located on, so that users can see where
> the file is located.

I suspect that even for XFS, we'd return a special cookie to say "On
data device #X", "on real time device #Y" or "on subvolume #Z"
rather than an actual block device. That will have a lot more
meaning to the XFS filesystem utilities that might use this
information than a raw block device (which may or may not exist!)
because then they don't have to jump through hoops to convert it to
something meaningful....

> > Darrick said it was useful for _inside_ the kernel.  How is it useful
> > for outside the kernel?
> 
> In my experience, this can be very useful for users to understand how their
> file is allocated if there are performance or other issues with a particular
> device.

*nod*

And when you have allocation policies that select different
devices for different files it makes it possible to easily verify
the policy is working correctly.

https://patchwork.kernel.org/patch/10081163/


> Also, in some respects, it is _required_ for multi-device filesystems,
> since it makes it clear that block 123 on one device is not related to the same
> block number on a different device.

*nod*

On xfs, we have to do 'xfs_io -c stat -c "fiemap -v" <file>' to
get the RT dev attribute in addition to the extent list right now.
It would be good to get them in just the fiemap call.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com