Re: [PATCH] [RFC] iomap: Use FUA for pure data O_DSYNC DIO writes

["Theodore Ts'o" <tytso@mit.edu>]

On Thu, Mar 01, 2018 at 12:41:44PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
> 
> 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@quack2.suse.cz/

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