On Tue, Jan 14, 2020 at 04:48:29PM +0000, David Howells wrote: > Again with regard to my rewrite of fscache and cachefiles: > > https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/log/?h=fscache-iter > > I've got rid of my use of bmap()! Hooray! > > However, I'm informed that I can't trust the extent map of a backing file to > tell me accurately whether content exists in a file because: > > (a) Not-quite-contiguous extents may be joined by insertion of blocks of > zeros by the filesystem optimising itself. This would give me a false > positive when trying to detect the presence of data. > > (b) Blocks of zeros that I write into the file may get punched out by > filesystem optimisation since a read back would be expected to read zeros > there anyway, provided it's below the EOF. This would give me a false > negative. > > Is there some setting I can use to prevent these scenarios on a file - or can > one be added? I don't think there's any way to do this in a portable way, at least today. There is a hack we could be use that would work for ext4 today, at least with respect to (a), but I'm not sure we would want to make any guarantees with respect to (b). I suspect I understand why you want this; I've fielded some requests for people wanting to do something very like this at $WORK, for what I assume to be for the same reason you're seeking to do this; to create do incremental caching of files and letting the file system track what has and hasn't been cached yet. If we were going to add such a facility, what we could perhaps do is to define a new flag indicating that a particular file should have no extent mapping optimization applied, such that FIEMAP would return a mapping if and only if userspace had written to a particular block, or had requested that a block be preallocated using fallocate(). The flag could only be set on a zero-length file, and this might disable certain advanced file system features, such as reflink, at the file system's discretion; and there might be unspecified performance impacts if this flag is set on a file. File systems which do not support this feature would not allow this flag to be set. - Ted
