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iomap: Use FUA for pure data O_DSYNC DIO writes
If we are doing direct IO writes with datasync semantics, we often have to flush metadata changes along with the data write. However, if we are overwriting existing data, there are no metadata changes that we need to flush. In this case, optimising the IO by using FUA write makes sense. We know from the IOMAP_F_DIRTY flag as to whether a specific inode requires a metadata flush - this is currently used by DAX to ensure extent modification as stable in page fault operations. For direct IO writes, we can use it to determine if we need to flush metadata or not once the data is on disk. Hence if we have been returned a mapped extent that is not new and the IO mapping is not dirty, then we can use a FUA write to provide datasync semantics. This allows us to short-cut the generic_write_sync() call in IO completion and hence avoid unnecessary operations. This makes pure direct IO data write behaviour identical to the way block devices use REQ_FUA to provide datasync semantics. On a FUA enabled device, a synchronous direct IO write workload (sequential 4k overwrites in 32MB file) had the following results: # xfs_io -fd -c "pwrite -V 1 -D 0 32m" /mnt/scratch/boo kernel time write()s write iops Write b/w ------ ---- -------- ---------- --------- (no dsync) 4s 2173/s 2173 8.5MB/s vanilla 22s 370/s 750 1.4MB/s patched 19s 420/s 420 1.6MB/s The patched code clearly doesn't send cache flushes anymore, but instead uses FUA (confirmed via blktrace), and performance improves a bit as a result. However, the benefits will be higher on workloads that mix O_DSYNC overwrites with other write IO as we won't be flushing the entire device cache on every DSYNC overwrite IO anymore. Signed-Off-By:Dave Chinner <dchinner@redhat.com> Reviewed-by: