Hi, Our team in Microsoft, which works on the Linux SMB3 client kernel filesystem has recently been exploring the use of fscache on top of ext4 for caching the network filesystem data for some customer workloads. However, the maintainer of fscache (David Howells) recently warned us that a few other extent based filesystem developers pointed out a theoretical bug in the current implementation of fscache/cachefiles. It currently does not maintain a separate metadata for the cached data it holds, but instead uses the sparseness of the underlying filesystem to track the ranges of the data that is being cached. The bug that has been pointed out with this is that the underlying filesystems could bridge holes between data ranges with zeroes or punch hole in data ranges that contain zeroes. (@David please add if I missed something). David has already begun working on the fix to this by maintaining the metadata of the cached ranges in fscache itself. However, since it could take some time for this fix to be approved and then backported by various distros, I'd like to understand if there is a potential problem in using fscache on top of ext4 without the fix. If ext4 doesn't do any such optimizations on the data ranges, or has a way to disable such optimizations, I think we'll be okay to use the older versions of fscache even without the fix mentioned above. Opinions? -- Regards, Shyam
