Re: [patch] reduce log I/O latency V2

[Lachlan McIlroy <lachlan@xxxxxxx>]

Looks good Dave.  Shouldn't we also tag the second buffer in the
split log write case?

David Chinner wrote:
> Reduce log I/O latency
>
> To ensure that log I/O is issued as the highest priority I/O, set
> the I/O priority of the log I/O to the highest possible. This will
> ensure that log I/O is not held up behind bulk data or other
> metadata I/O as delaying log I/O can pause the entire transaction
> subsystem. Introduce a new buffer flag to allow us to tag the log
> buffers so we can discriminate when issuing the I/O.
>
> Version 2:
> - don't use realtime priority class
>
> Signed-off-by: Dave Chinner <dgc@xxxxxxx>
> ---
>  fs/xfs/linux-2.6/xfs_buf.c |    3 +++
>  fs/xfs/linux-2.6/xfs_buf.h |    5 ++++-
>  fs/xfs/xfs_log.c           |    2 ++
>  3 files changed, 9 insertions(+), 1 deletion(-)
>
> Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_buf.c       2007-12-19 
> 01:39:42.286538135 +1100
> +++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.c    2007-12-19 11:56:47.977477993 
> +1100
> @@ -1198,6 +1198,9 @@ next_chunk:
>  
>  submit_io:
>         if (likely(bio->bi_size)) {
> +               /* log I/O should not be delayed by anything except realtime. */
> +               if (bp->b_flags & XBF_LOG_BUFFER)
> +                       bio_set_prio(bio, IOPRIO_PRIO_VALUE(IOPRIO_CLASS_BE, 
> 0));
>                 submit_bio(rw, bio);
>                 if (size)
>                         goto next_chunk;
> Index: 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.h
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/linux-2.6/xfs_buf.h       2007-12-19 
> 01:11:16.871181483 +1100
> +++ 2.6.x-xfs-new/fs/xfs/linux-2.6/xfs_buf.h    2007-12-19 11:55:09.642183801 
> +1100
> @@ -53,7 +53,8 @@ typedef enum {
>         XBF_DELWRI = (1 << 6),  /* buffer has dirty pages                  */
>         XBF_STALE = (1 << 7),     /* buffer has been staled, do not find it  */
>         XBF_FS_MANAGED = (1 << 8),  /* filesystem controls freeing memory  */
> -       XBF_ORDERED = (1 << 11),    /* use ordered writes            */
> +       XBF_LOG_BUFFER = (1 << 9),  /* Buffer issued by the log              */
> +       XBF_ORDERED = (1 << 11),    /* use ordered writes            */
>         XBF_READ_AHEAD = (1 << 12), /* asynchronous read-ahead               */
>  
>  	/* flags used only as arguments to access routines */
> @@ -339,6 +340,8 @@ extern void xfs_buf_trace(xfs_buf_t *, c
>  #define XFS_BUF_TARGET(bp)             ((bp)->b_target)
>  #define XFS_BUFTARG_NAME(target)       xfs_buf_target_name(target)
>  
> +#define XFS_BUF_SET_LOGBUF(bp)	((bp)->b_flags |= XBF_LOG_BUFFER)
> +
>  static inline int xfs_bawrite(void *mp, xfs_buf_t *bp)
>  {
>         bp->b_fspriv3 = mp;
> Index: 2.6.x-xfs-new/fs/xfs/xfs_log.c
> ===================================================================
> --- 2.6.x-xfs-new.orig/fs/xfs/xfs_log.c 2007-12-19 01:11:16.871181483 +1100
> +++ 2.6.x-xfs-new/fs/xfs/xfs_log.c      2007-12-19 11:55:09.698176566 +1100
> @@ -1443,6 +1443,8 @@ xlog_sync(xlog_t          *log,
>         XFS_BUF_ZEROFLAGS(bp);
>         XFS_BUF_BUSY(bp);
>         XFS_BUF_ASYNC(bp);
> +       XFS_BUF_SET_LOGBUF(bp);
> +
>         /*
>          * Do an ordered write for the log block.
>          * Its unnecessary to flush the first split block in the log wrap case.