Andreas Dilger <adilger@xxxxxxxxx> wrote: > > Would you like to explain why you want to know such fs internal info? > > I believe David wants it to store sparse files as an cache and use FIEMAP to > determine if the blocks are cached locally, or if they need to be fetched from > the server. If the filesystem doesn't store the written blocks accurately, > there is no way for the local cache to know whether it is holding valid data > or not. More or less. I have no particular attachment to bmap or FIEMAP as the interface to use. I'm just interested in finding out quickly if the data I want is present. If call_read_iter() will return a short read on hitting a hole, I can manage if I can find out if just the first byte is present. Finding out if the block is present allows me to avoid shaping read requests from VM readahead into 256k blocks - which may require the allocation of extra pages for bufferage. David
