Re: Extending FIEMAP ioctl to report device id

[Nikolay Borisov <nborisov@suse.com>]

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.



> 
> 
> 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
>