Hi all, bcachefs has a nodataio mount option that is used for isolated metadata performance testing purposes. When enabled, it performs all metadata I/O as normal and shortcuts data I/O by directly invoking bio completion. Kent had asked for something similar for fs comparison purposes some time ago and I put together a quick hack based around an iomap flag and mount option for XFS. I don't recall if I ever posted the initial version and Kent recently asked about whether we'd want to consider merging something like this. I think there are at least a couple things that probably need addressing before that is a viable option. One is that the mount option is kind of hacky in and of itself. Beyond that, this mechanism provides a means for stale data exposure because writes with nodataio mode enabled will operate as if writes were completed normally (including unwritten extent conversion). Therefore, a remount to !nodataio mode means we read off whatever was last written to storage. Kent mentioned that Eric (or somebody?) had floated the idea of a mkfs time feature flag or some such to control nodataio mode. That would avoid mount api changes in general and also disallow use of such filesystems in a non-nodataio mode, so to me seems like the direction bcachefs should go with its variant of this regardless. Personally, I don't have much of an opinion on whether something like this lands upstream or just remains as a local test hack for isolated performance testing. The code is simple enough as it is and not really worth the additional polishing for the latter, but I offered to at least rebase and post for discussion. Thoughts, reviews, flames appreciated. Brian Brian Foster (3): iomap: factor out a bio submission helper iomap: add nosubmit flag to skip data I/O on iomap mapping xfs: add nodataio mount option to skip all data I/O fs/iomap/buffered-io.c | 37 ++++++++++++++++++++++++++++--------- fs/xfs/xfs_iomap.c | 3 +++ fs/xfs/xfs_mount.h | 2 ++ fs/xfs/xfs_super.c | 6 +++++- include/linux/iomap.h | 1 + 5 files changed, 39 insertions(+), 10 deletions(-) -- 2.44.0
