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SubjectRe: Re: Why blktrace didn't trace requests merge?
On 2012-09-18 13:49 Jens Axboe <axboe@kernel.dk> Wrote:
>On 2012-09-18 02:30, Jianpeng Ma wrote:
>> On 2012-09-18 02:27 Jens Axboe <axboe@kernel.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?
>
>It is bio to request, correct. Request to request merges are relatively
>more rare.
>
>--
>Jens Axboe
>
Thanks very much, I know.

Jianpeng
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