On Thu, Mar 01, 2018 at 12:41:44PM +1100, Dave Chinner wrote: > From: Dave Chinner <dchinner@xxxxxxxxxx> > > 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 teh IOMAP_F_DIRTY flag as to whether a specific inode > requires a metadata flush - this is currently used by DAX to ensure > extent modi$fication 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. Hi, I've gotten an inquiry from some engineers at Microsoft who would really like it if ext4 could use FUA writes when doing O_DSYNC writes, since this is soemthing that SQL Server uses. In the discussion for this patch series back in 2018[1], ext4 hadn't yet converted over to iomap for Direct I/O, and so adding this feature for ext4 wasn't really practical. [1] https://lore.kernel.org/all/20180319160650.mavedzwienzgwgqi@xxxxxxxxxxxxxx/ Today, ext4 does use iomap for DIO, but an experiment seems to indicate that something hasn't been wired up to enable FUA for O_DSYNC writes. I've looked at fs/iomap/direct-io.c and it wasn't immediately obvious what I need to add to enable this feature. I was wondering if you could me some quick hints about what and where I should be looking? Many thanks! - Ted
