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. 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
