[ adding Robert who had a question about this patch, and Jan + Ted for
the ext4 implications ]
I talked with Robert offline and he had some questions about this
patch that belong on the mailing list.
On Wed, Feb 28, 2018 at 5:41 PM, Dave Chinner <david@xxxxxxxxxxxxx> 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.
>
> 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.
>
> Now that iomap_dio_rw() is determining if REQ_FUA can be used, we
> have to stop issuing generic_write_sync() calls from the XFS code
> when REQ_FUA is issued, otherwise it will still throw a cache flush
> to the device via xfs_file_fsync(). To do this, we need to make
> iomap_dio_rw() always responsible for issuing generic_write_sync()
> when necessary, not just for AIO calls. This means the filesystem
> doesn't have to guess when cache flushes are necessary now.
>
> 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@xxxxxxxxxx>
> ---
> fs/iomap.c | 38 +++++++++++++++++++++++++++++++++-----
> fs/xfs/xfs_file.c | 5 +++++
> 2 files changed, 38 insertions(+), 5 deletions(-)
>
> diff --git a/fs/iomap.c b/fs/iomap.c
> index afd163586aa0..bcc90e3a2e3f 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -685,6 +685,7 @@ EXPORT_SYMBOL_GPL(iomap_seek_data);
> * Private flags for iomap_dio, must not overlap with the public ones in
> * iomap.h:
> */
> +#define IOMAP_DIO_WRITE_FUA (1 << 29)
> #define IOMAP_DIO_WRITE (1 << 30)
> #define IOMAP_DIO_DIRTY (1 << 31)
>
> @@ -760,8 +761,19 @@ static ssize_t iomap_dio_complete(struct iomap_dio *dio)
> }
>
> inode_dio_end(file_inode(iocb->ki_filp));
> - kfree(dio);
>
> + /*
> + * If a FUA write was done, then that is all we required for datasync
> + * semantics -. we don't need to call generic_write_sync() to complete
> + * the write.
> + */
> + if (ret > 0 &&
> + (dio->flags & (IOMAP_DIO_WRITE|IOMAP_DIO_WRITE_FUA)) ==
> + IOMAP_DIO_WRITE) {
> + ret = generic_write_sync(iocb, ret);
> + }
> +
> + kfree(dio);
> return ret;
> }
>
> @@ -769,12 +781,9 @@ static void iomap_dio_complete_work(struct work_struct *work)
> {
> struct iomap_dio *dio = container_of(work, struct iomap_dio, aio.work);
> struct kiocb *iocb = dio->iocb;
> - bool is_write = (dio->flags & IOMAP_DIO_WRITE);
> ssize_t ret;
>
> ret = iomap_dio_complete(dio);
> - if (is_write && ret > 0)
> - ret = generic_write_sync(iocb, ret);
> iocb->ki_complete(iocb, ret, 0);
> }
>
> @@ -883,6 +892,15 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> dio->flags |= IOMAP_DIO_COW;
> if (iomap->flags & IOMAP_F_NEW)
> need_zeroout = true;
> + /*
> + * Use a FUA write if we need datasync semantics and this is a
> + * pure data IO that doesn't require any metadata updates. This
> + * allows us to avoid cache flushes on IO completion.
> + */
> + else if (!(iomap->flags & (IOMAP_F_SHARED|IOMAP_F_DIRTY)) &&
> + (dio->flags & IOMAP_DIO_WRITE) &&
> + (dio->iocb->ki_flags & IOCB_DSYNC))
> + dio->flags |= IOMAP_DIO_WRITE_FUA;
> break;
> default:
> WARN_ON_ONCE(1);
> @@ -930,7 +948,11 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
>
> n = bio->bi_iter.bi_size;
> if (dio->flags & IOMAP_DIO_WRITE) {
> - bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC | REQ_IDLE);
> + int op_flags = REQ_SYNC | REQ_IDLE;
> +
> + if (dio->flags & IOMAP_DIO_WRITE_FUA)
> + op_flags |= REQ_FUA;
> + bio_set_op_attrs(bio, REQ_OP_WRITE, op_flags);
> task_io_account_write(n);
> } else {
> bio_set_op_attrs(bio, REQ_OP_READ, 0);
> @@ -961,6 +983,12 @@ iomap_dio_actor(struct inode *inode, loff_t pos, loff_t length,
> return copied;
> }
>
> +/*
> + * iomap_dio_rw() always completes O_[D]SYNC writes regardless of whether the IO
> + * is being issued as AIO or not. This allows us to optimise pure data writes to
> + * use REQ_FUA rather than requiring generic_write_sync() to issue a REQ_FLUSH
> + * post write.
> + */
> ssize_t
> iomap_dio_rw(struct kiocb *iocb, struct iov_iter *iter,
> const struct iomap_ops *ops, iomap_dio_end_io_t end_io)
> diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> index 260ff5e5c264..81aa3b73471e 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -732,6 +732,11 @@ xfs_file_write_iter(
> ret = xfs_file_dio_aio_write(iocb, from);
> if (ret == -EREMCHG)
> goto buffered;
> + /*
> + * Direct IO handles sync type writes internally on I/O
> + * completion.
> + */
> + return ret;
> } else {
> buffered:
> ret = xfs_file_buffered_aio_write(iocb, from);
> --
> 2.16.1
>
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