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

To: David Chinner <dgc@xxxxxxx>
Subject: Re: [patch] reduce log I/O latency V2
From: Lachlan McIlroy <lachlan@xxxxxxx>
Date: Tue, 22 Jan 2008 14:42:13 +1100
Cc: xfs-dev <xfs-dev@xxxxxxx>, xfs-oss <xfs@xxxxxxxxxxx>
In-reply-to: <20080121041649.GA155259@xxxxxxx>
References: <20080121041649.GA155259@xxxxxxx>
Reply-to: lachlan@xxxxxxx
Sender: xfs-bounce@xxxxxxxxxxx
User-agent: Thunderbird 2.0.0.9 (X11/20071031)
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.



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