Re: Why blktrace didn't trace requests merge?

[Jens Axboe]

On 2012-09-18 02:30, Jianpeng Ma wrote:
> On 2012-09-18 02:27 Jens Axboe <axboe@xxxxxxxxx> 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?

It is bio to request, correct. Request to request merges are relatively
more rare.

-- 
Jens Axboe

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