On Thu, 2007-04-12 at 05:05 -0600, Andreas Dilger wrote:
> I'm interested in getting input for implementing an ioctl to efficiently
> map file extents & holes (FIEMAP) instead of looping over FIBMAP a billion
> times. We already have customers with single files in the 10TB range and
> we additionally need to get the mapping over the network so it needs to
> be efficient in terms of how data is passed, and how easily it can be
> extracted from the filesystem.
>
> I had come up with a plan independently and was also steered toward
> XFS_IOC_GETBMAP* ioctls which are in fact very similar to my original
> plan, though I think the XFS structs used there are a bit bloated.
>
> There was also recent discussion about SEEK_HOLE and SEEK_DATA as
> implemented by Sun, but even if we could skip the holes we still might
> need to do millions of FIBMAPs to see how large files are allocated
> on disk. Conversely, having filesystems implement an efficient FIBMAP
> ioctl (or ->fiemap() method) could in turn be leveraged for SEEK_HOLE
> and SEEK_DATA instead of doing looping over ->bmap() inside the kernel
> as I saw one patch.
>
I certainly hope not. SEEK_HOLE/SEEK_DATA is a poor interface and
doesn't deserve to spread.
OTOH, this is nicely done.
>
> struct fibmap_extent {
> __u64 fe_start; /* starting offset in bytes */
> __u64 fe_len; /* length in bytes */
> }
>
> struct fibmap {
> struct fibmap_extent fm_start; /* offset, length of desired mapping */
> __u32 fm_extent_count; /* number of extents in array */
> __u32 fm_flags; /* flags (similar to XFS_IOC_GETBMAP) */
> __u64 unused;
> struct fibmap_extent fm_extents[0];
> }
>
> #define FIEMAP_LEN_MASK 0xff000000000000
> #define FIEMAP_LEN_HOLE 0x01000000000000
> #define FIEMAP_LEN_UNWRITTEN 0x02000000000000
>
--
Nicholas Miell <nmiell@xxxxxxxxxxx>
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