Re: [Question] How to perform stride access?

[Jens Axboe <axboe@kernel.dk>]

On 2014-09-28 04:36, Sitsofe Wheeler wrote:
> (Resending because first mail had an HTML part)
>
> On 28 September 2014 03:24, Jens Axboe <axboe@kernel.dk> wrote:
>>> On 24 September 2014 10:52, Sitsofe Wheeler <sitsofe@gmail.com> wrote:
>>>>
>>>> This looks like a bug. I can reproduce it with 2.1.11-11-gb7f5 too:
>>>>
>>>> dd if=/dev/zero of=/dev/shm/1M bs=1M count=1
>>>> fio --bs=4k --rw=write:4k --filename=/dev/shm/1M --stonewall --name=1M
>>>> --io_limit=1M  --name=2M --io_limit=2M
>>>> [...]
>>>>
>>>> Run status group 0 (all jobs):
>>>>     WRITE: io=512KB, aggrb=256000KB/s, minb=256000KB/s, maxb=256000KB/s,
>>>> mint=2msec, maxt=2msec
>>>>
>>>> Run status group 1 (all jobs):
>>>>     WRITE: io=512KB, aggrb=256000KB/s, minb=256000KB/s, maxb=256000KB/s,
>>>> mint=2msec, maxt=2msec
>>>>
>>>> Why isn't io 1024KB for group 0? Additionally, shouldn't the total io
>>>> written each group be different? Jens?
>>
>> You are doing a sequential workload, skipping 4k every time. First write
>> will be to offset 0, next to 8KB, etc. Write 128 would be to 1040384,
> which
>> is 1MB - 8KB. Hence the next feasible offset after that would be 1MB,
> which
>> is end of the file. So how could it do more than 512KB of IO? That's 128 *
>> 4KB.
>>
>> I didn't read the whole thread in detail, just looked at your last example
>> here. And for that one, I don't see anything wrong.
>
> I guess I would have thought io_limit always forced wraparound. For example:
>
> # dd if=/dev/zero of=/dev/shm/1M bs=1M count=1
> # fio --bs=4k --filename=/dev/shm/1M --name=go1 --rw=write
> [...]
> Run status group 0 (all jobs):
>    WRITE: io=1024KB, aggrb=341333KB/s, minb=341333KB/s, maxb=341333KB/s,
> mint=3msec, maxt=3msec
> # fio --bs=4k --filename=/dev/shm/1M --name=go2 --io_limit=2M --rw=write
> Run status group 0 (all jobs):
>    WRITE: io=2048KB, aggrb=341333KB/s, minb=341333KB/s, maxb=341333KB/s,
> mint=6msec, maxt=6msec
> [...]
> # fio --bs=4k --filename=/dev/shm/1M --name=go3 --io_limit=2M --rw=write:4k
> [...]
>    WRITE: io=512KB, aggrb=256000KB/s, minb=256000KB/s, maxb=256000KB/s,
> mint=2msec, maxt=2msec
> Run status group 0 (all jobs):
> # fio --bs=4k --filename=/dev/shm/1M --name=go4 --io_limit=2M --rw=write:4k
> [...]
> Run status group 0 (all jobs):
>    WRITE: io=512KB, aggrb=256000KB/s, minb=256000KB/s, maxb=256000KB/s,
> mint=2msec, maxt=2msec
>
> go2 is a plain sequential job that does twice as much I/O as go1. Given
> that the size of the file being written to has not changed between the runs
> one could guess that fio simply wrapped around and started from the first
> offset (0) to write the second MB of data. Given this isn't it a fair
> assumption that when doing a skipping workload if io_limit is used (as in
> go4) and an offset beyond the end of the device is produced the same
> wraparound behaviour as go2 should occur and the total io done should match
> that specified in io_limit?

I would agree on that, behavior for those cases _should_ be the same. 
Without the holed IO, it closes/reopens the file and repeats the 1M 
writes. With it, it does not. I will take a look.

-- 
Jens Axboe