On Tue, Nov 17, 2020 at 12:15:26PM -0500, Theodore Y. Ts'o wrote: > What is the expected use case for Direct I/O using fscrypt? This > isn't a problem which is unique to fscrypt, but one of the really > unfortunate aspects of the DIO interface is the silent fallback to > buffered I/O. We've lived with this because DIO goes back decades, > and the original use case was to keep enterprise databases happy, and > the rules around what is necessary for DIO to work was relatively well > understood. > > But with fscrypt, there's going to be some additional requirements > (e.g., using inline crypto) required or else DIO silently fall back to > buffered I/O for encrypted files. Depending on the intended use case > of DIO with fscrypt, this caveat might or might not be unfortunately > surprising for applications. > > I wonder if we should have some kind of interface so we can more > explicitly allow applications to query exactly what the requirements > might be for a particular file vis-a-vis Direct I/O. What are the > memory alignment requirements, what are the file offset alignment > requirements, what are the write size requirements, for a particular > file. In Ye Olde days there was XFS_IOC_DIOINFO to communicate all that (xfs hardcodes 512b file offset alignment), but in this modern era perhaps it's time to shovel that into statx... --D > > - Ted
