Re: Extending FIEMAP ioctl to report device id

[Carlos Maiolino <cmaiolino@redhat.com>]

Hi,

On Mon, Feb 11, 2019 at 01:29:57PM +0200, Nikolay Borisov wrote:
> 
> 
> On 11.02.19 г. 11:43 ч., Carlos Maiolino wrote:
> > Hi.
> > 
> > A discussion has been started on another thread [1], with the idea of extending
> > FIEMAP ioctl interface, to also report the device id where the extents being
> > reported are physically located. I've started to work on the extension, but,
> > before I spend time implementing it, I'd rather start a discussion to ensure
> > it's really feasible or just a waste of time in pursuing it.
> > 
> > The whole context, can be found in the thread [1], more specifically in the
> > discussion started on patch 9, here [2].
> > 
> > About the proposal:
> > 
> > - The general idea, is to provide a way for FIEMAP ioctls to return the device
> >   id where each extent is physically located.
> > - This is particularly useful for those filesystems where the file extents are
> >   located on a different block device other than that associated with the
> >   superblock , for example, btrfs using multiple devices, and XFS when using a
> >   real-time device.
> > 
> > Achieving this is relatively easy, using one of the __u32 fe_reserved fields in
> > struct fiemap_extent, to create a new field (__u32 fe_device), which can be used
> > for two purposes, based on two new FIEMAP_EXTENT_ flags : 
> > 
> > - FIEMAP_EXTENT_DEVICE: which will indicate the fiemap_extent.fe_device contains
> >   the major/minor numbers of the block device where the specific extent is
> >   located
> > 
> > - FIEMAP_EXTENT_COOKIE (of _EXTENT_PRIVATE), which indicates the
> >   fiemap_extent.fe_device will contain a special meaning depending on the fs.
> >   Such flag sounded interesting for distributed filesystems, which could use
> >   this field for example, to specify each node of the cluster (or whatever other
> >   name is defined by the specific fs) that specific extent is located.
> 
> Who decides which flag is set? Do you intend for the default behavior to
> be FIEMAP_EXTENT_DEVICE which could be overridden by
> FIEMAP_EXTENT_COOKIE? IMHO a more becoming name could be
> FIEMAP_EXTENT_DEV_PRIVATE or PRIVATE_DEV.
> 

The idea is:

- If none of the flags are set, the fe_device field is ignored by FIEMAP
  infrastructure.
- If FIEMAP_EXTENT_DEVICE is set, and _COOKIE (or _PRIVATE) IS NOT, then, the
  fe_device is the major/minor dev_t of the block device holding the extent.
- If both flags ARE SET, the fe_device holds a value that only the specific
  filesystem (and maybe its users) can properly interpret.


> 
> 
> > 
> > 
> > As mentioned before, implementing it, looks not that difficult, considering such
> > reserved fields are not to be touched by userspace, and using one of the new
> > fields won't break any current userspace application which doesn't understand
> > the new data.
> > But still, things which are worth to discuss is if such information (the
> > physical location of the extents) is something that should be exported to
> > userspace or not.
> > 
> > Any comments if this is something worth to implement or not, are welcome.
> > 
> > Cheers
> > 
> > [1] https://www.spinics.net/lists/linux-fsdevel/msg136559.html
> > [2] https://www.spinics.net/lists/linux-fsdevel/msg136568.html
> > 

-- 
Carlos