Re: [Question] What if the parameter for --size option is smaller than the target device?

[Akira Hayakawa <ruby.wktk@gmail.com>]

Thanks Sitsofe,
it starts to submit 32M in total.

- Akira

On 9/21/14 7:21 PM, Sitsofe Wheeler wrote:
> Hi,
> 
> On 21 September 2014 04:57, Akira Hayakawa <ruby.wktk@gmail.com> wrote:
>> I am writing a test with fio tool (2.0.8) and I am facing a problem.
>>
>> I just want to submit 512B random write to a block device.
>> In my case, the size of the device is 2MB and the amount of writes (specified by --size option) is 32MB.
> 
> You probably wanted the behaviour of io_limit
> (http://git.kernel.dk/?p=fio.git;a=blob;f=HOWTO;h=73e58ff695c3384df30094d6c15ebe9ed615460f;hb=HEAD#l433
> ) rather than size...
> 
>> like this (A line from device-mapper-test-suite. Is my command wrong?):
>>       ProcessControl.run("fio --name=test --filename=#{dev.path} --rw=randwrite --ioengine=libaio --direct=1 --size=#{@param[0]}m --bs=512")
>>
>> The problem is, the runtime is too short.
>> The block device is just a single HDD so, it will be 30sec or so. But it's actually 3sec...
>>
>> My guess is that fio uses min(device size, param to --size) as the amount of writes here.
> 
> That's correct in this context - size is limiting the last possible
> block that can be written to (see
> http://git.kernel.dk/?p=fio.git;a=blob;f=HOWTO;h=73e58ff695c3384df30094d6c15ebe9ed615460f;hb=HEAD#l421
> for details).
> 
>> To investigate it, I increased the size of the device to x10 and saw the runtime gets x10.
>>
>> My questions are:
>> 1. Is my guess correct?
> 
> Yes.
> 
>> 2. What shall I do to make fio work as I want it to do?
> 
> Use io_limit if your fio is new enough or number_ios
> (http://git.kernel.dk/?p=fio.git;a=blob;f=HOWTO;h=73e58ff695c3384df30094d6c15ebe9ed615460f;hb=HEAD#l778
> ).
> 
> Sitsofe | http://sucs.org/~sits/
>