Multiple jobs and fill_device

Hello list,

I'm wondering how the configuration below is interpreted:

    [global]
    ioengine=libaio
    direct=1
    loops=1
    numjobs=8
    
    [write-prep]
    rw=write
    filename=/dev/sdb
    iodepth=16
    bs=16k
    fill_device=1
    stonewall=1

Will every one of the 8 jobs try to fill the device completely or will they split the task of filling the device between themselves, where each will do only 1/8 of filling the device? My reading of documentation seems to suggest the device will be filled 8 times, with jobs, possibly, stepping on each other's toes (i.e. contesting the same block), but I'm not sure.

Best.

Oleg

Hi,

Yes the device will be filled but not due to fill_device=1. Confusingly fill_device is actually meant for when you are creating files on filesystems where you want the file to use up all the remaining space
(https://fio.readthedocs.io/en/latest/fio_doc.html#cmdoption-arg-fill-device
says this a different way). A file representing a device will already have a fixed size that can't be grown by just writing to it (OK there's thin provisioning but even there the "device" has a fixed size ahead of time). When a file already has a size that is what is used when "size" is not set (see
https://fio.readthedocs.io/en/latest/fio_man.html#cmdoption-arg-size
).

No they will all overwrite/overlap the same areas.

Your reading is correct - they will step on each others toes. If you want to somehow avoid this you'll have to look into offset_increment
(https://fio.readthedocs.io/en/latest/fio_man.html#cmdoption-arg-offset-increment
) and size (https://fio.readthedocs.io/en/latest/fio_man.html#cmdoption-arg-size
).

NB: stonewall=1 likely won't do anything in this case because no job definition is "after" another when using numjobs within the same job.

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