On Mon, Nov 04, 2013 at 10:14:42AM +1100, Dave Chinner wrote: > > FIEMAP is not the correct interface for data modifying operations. > It is an interface that returns information about file metadata (i.e > the layout of a file) - it is not an interface for modifying the > contents of the file. Well, it's been argued that FIEMAP was designed to be extensible, and we should use it for other operations. Where the bounds of that stretches to is certainly an arguable point, and I can understand your observation that something which causes a change to the contents of the file might not be best choice. > fallocate() is the interface used to modify file layout and > manipulate the data within it and hence, IMO, is much better aligned > to this operation. And to tell the truth, I'd much prefer such an > interface is guaranteed to return zeros to users rather than rely on > whether the underlying device supports discard or whatever they > return after a discard. i.e. if the user is asking to destroy the > data in the file, we better be able to ensure the data in the file > is in a known state at the filesystem level regardless of the > underlying storage capabilities.... There are two different things that a user might want to do: * write zeros * signal to the flash that the contents of the file are not needed (There's also a "secure discard" which recently got added which apparently adds a guarantee that for certain storage devices, all of the previous locations on the flash which had been mapped by the FTL would also get discarded --- from what I can tell this was some kind of eMMC thing, but I'll ignore this for now.) The specific feature request that I was given was to be a file-level equivalent of the BLKDISCARD ioctl --- and in the patch I added support for the BLKDISCARD ioctl to be applied to files, since that was the desired request. For other use cases, I agree that a file-level equivalent to the BLKZEROOUT ioctl might be more appropriate --- but that's not what would be most useful for the particular user application that I'm trying to support. (In fact, we're using flash where BLKDISCARDZEROS returns false, since the discard might be a no-op under certain power fail scenarios, since the flash doesn't force a stable write of the FTL metadata when it receives a discard command, for performance reasons.) The reason why I found FIEMAP to more convenient from an implementation perspective is that unlike fallocate(), we're not actually modifying the logical->physical block map of the file. We just need to be able to ask the file system to iterate over the extents in a particular logical block range. And FIEMAP does that, most conveniently, where as if we used fallocate(), each file system would have to implement the code to issue the discard for the relevant extent ranges in fs specific code --- or we would have to reimplement the FIEMAP machinery in a more general fashion so that we could call sb_issue_discard (or sb_issue_zeroout) from the VFS layer, after receiving the extent ranges from the fs-specific layer. Regards, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
