linux-kernel - Re: Re: Why blktrace didn't trace requests merge?

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Date:	Tue, 18 Sep 2012 08:30:20 +0800
From:	"Jianpeng Ma" <majianpeng@...il.com>
To:	axboe <axboe@...nel.dk>, tj <tj@...nel.org>
Cc:	rostedt <rostedt@...dmis.org>, fweisbec <fweisbec@...il.com>,
	mingo <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Li Zefan" <lizf@...fujitsu.com>, "Tao Ma" <boyu.mt@...bao.com>
Subject: Re: Re: Why blktrace didn't trace requests merge?

On 2012-09-18 02:27 Jens Axboe <axboe@...nel.dk> Wrote:
>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
>
First, Thanks your time!
If i understand correctly, the merge of your example is bio with request, not request wiht request.
Yes or no?

Thanks!
Jianpeng