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Date: Mon, 17 Sep 2012 20:27:58 +0200
From: Jens Axboe <axboe@kernel.dk>
MIME-Version: 1.0
To: Tejun Heo <tj@kernel.org>
CC: Steven Rostedt <rostedt@goodmis.org>, Jianpeng Ma <majianpeng@gmail.com>,
        fweisbec <fweisbec@gmail.com>, mingo <mingo@redhat.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Li Zefan <lizf@cn.fujitsu.com>, Tao Ma <boyu.mt@taobao.com>
Subject: Re: Why blktrace didn't trace requests merge?
References: <201209171933464847623@gmail.com> <1347888148.13080.20.camel@gandalf.local.home> <20120917175534.GG18677@google.com>
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On 2012-09-17 19:55, Tejun Heo wrote:
> (cc'ing Jens)
> 
> On Mon, Sep 17, 2012 at 09:22:28AM -0400, Steven Rostedt wrote:
>> On Mon, 2012-09-17 at 19:33 +0800, Jianpeng Ma wrote:
>>> Hi all:
>>> 	I used blktrace to trace some io.But i can't find requests merge. I searched the code and did't not find.
>>> 	Why? 
>>> 									
>>
>> No idea. I don't use blktrace much, but I Cc'd those that understand it
>> better than I.

Works for me:

[...]


  8,0    0       26     0.009147735   664  A  WS 315226143 + 8 <- (8,7) 19406344
  8,0    0       27     0.009148677   664  Q  WS 315226143 + 8 [btrfs-submit-1]
  8,0    0       28     0.009152967   664  G  WS 315226143 + 8 [btrfs-submit-1]
  8,0    0       29     0.009154242   664  P   N [btrfs-submit-1]
  8,0    0       30     0.009155538   664  A  WS 315226151 + 8 <- (8,7) 19406352
  8,0    0       31     0.009155743   664  Q  WS 315226151 + 8 [btrfs-submit-1]
  8,0    0       32     0.009157086   664  M  WS 315226151 + 8 [btrfs-submit-1]
  8,0    0       33     0.009158716   664  I  WS 315226143 + 16 [btrfs-submit-1]

That's from a quick trace of /dev/sda. I started blktrace, then did:

$ dd if=/dev/zero of=foo bs=4k count=128 && sync

to ensure that I knew merges would be happening. Output stats at the end:

Total (sda):
 Reads Queued:           7,       44KiB  Writes Queued:         447,     7692KiB
 Read Dispatches:        7,       44KiB  Write Dispatches:      416,     7692KiB
 Reads Requeued:         0               Writes Requeued:         0
 Reads Completed:        7,       44KiB  Writes Completed:      435,     5864KiB
 Read Merges:            0,        0KiB  Write Merges:           23,      428KiB
 IO unplugs:            78               Timer unplugs:           0

-- 
Jens Axboe