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From: Sitsofe Wheeler <sitsofe@gmail.com>
Date: Tue, 13 Nov 2018 05:19:25 +0000
Message-ID: <CALjAwxg130qgxbCqV5Nhd=cef4FbTT7OKiXL7h6Fn0GiQUKGcQ@mail.gmail.com>
Subject: Re: Understanding offset modifier
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Sender: fio-owner@vger.kernel.org
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To: Mohanraj B <bmohanraj91@gmail.com>
Cc: fio <fio@vger.kernel.org>

Hi,

(You just restated your original question verbatim which unfortunately
doesn't move the problem forward. I was hoping you were going to point
out the exact specific part of the offset modifier concept you were
struggling with by using a worked example to show what you did
understand and which part you didn't so we could expand on just that.
I'll take a stab at explaining things this time but next time try and
be specific and briefly demonstrate what bits you did understand if
you're asked for clarification :-)

I'm just going to quote some text because "offset" in a different
context refers to another option:

>From https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-readwrite :
"It is possible to specify the number of I/Os to do before getting a
new offset by appending :<nr> to the end of the string given. For a
random read, it would look like rw=randread:8 for passing in an offset
modifier with a value of 8. If the suffix is used with a sequential
I/O pattern, then the <nr> value specified will be added to the
generated offset for each I/O turning sequential I/O into sequential
I/O with holes. For instance, using rw=write:4k will skip 4k for every
write. Also see the rw_sequencer option."

So in the above the "offset modifier/suffix" has two different meanings:
- With random I/O, the offset suffix is saying "do <nr> I/Os
sequentially before randomly* picking another starting point and then
doing another <nr> I/Os sequentially"
- With sequential I/O the offset suffix refers to the "distance to
skip between I/Os"

Seen another way, depending on context the offset modifier is
controlling the "the offset (distance) chosen from the first allowable
position for I/O before doing sequential I/O" or the "offset
(distance) from the end of the previous I/O".

(Note: I'm skipping over the effect of rw_sequencer in the above to
keep this discussion shorter)

As to your second question, for many jobs so long as I use the same
job options verification should work with an offset modifier. For
example:

fio --name=gappy_verify --size=512k --filename=/tmp/fio.tmp
--rw=write:4k --verify=crc32c

wrote every other 4k and then verified it. If you're splitting up your
verifies into separate jobs for some reason, you may find the
verify_only and do_verify options useful (see
https://fio.readthedocs.io/en/latest/fio_doc.html#verification ).

Does this help at all?

* This is a simplification but it gives you the idea.

On Tue, 13 Nov 2018 at 01:20, Mohanraj B <bmohanraj91@gmail.com> wrote:
>
> Hi,
>
> Thanks for the response.
>
> I have gone through and worked out the example.
> Below are my queries:
> what is offset?
> How to verify the results if used the :<nr> at the end of the string ?
>
> Thanks and Regards,
> Mohan
>
> On 11/7/18, Sitsofe Wheeler <sitsofe@gmail.com> wrote:
> > Hi,
> >
> > On Wed, 7 Nov 2018 at 11:31, Mohanraj B <bmohanraj91@gmail.com> wrote:
> >>
> >> Hello all,
> >>
> >>
> >> https://fio.readthedocs.io/en/latest/fio_man.html#i-o-type
> >>
> >> what is offset modifier, some please explain.
> >> It is possible to specify the number of I/Os to do before getting a
> >> new offset by appending :<nr> to the end of the string given.
> >
> > I guess you mean mean the rw <nr> parameter in the final paragraph in
> > https://fio.readthedocs.io/en/latest/fio_man.html#cmdoption-arg-readwrite
> > . Can you say what part of that paragraph and the rw_sequencer section
> > you are struggling to understand and did you work through the
> > randread:8 and rw=write:4k examples?
> >
> > --
> > Sitsofe | http://sucs.org/~sits/
> >

-- 
Sitsofe | http://sucs.org/~sits/