Re: fio file test patterns

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Perhaps I should describe what I'm seeing.  I have two terminals open.
 In one, I launch:
  $ watch "ls -l foo* | head -n 20"
In the other terminal I kick off fio.  fio first spits out the "Laying
out" messages, one per job.  Immediately the full 4MB files appear in
the directory listing.  *Then* fio starts writing to the files, as
indicated by the fio output.  The size of each file never changes
during the test.

I wasn't expecting the full 4Mb files to be created before the
sequential writes.  I expected to not see any files initially, and
then a file for each job growing at a rate of 200Kbs throughout the
test until reaching the 4MB limit.

Perhaps I'm just configuring fio incorrectly?

brian


On Wed, Aug 31, 2011 at 5:38 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> On 2011-08-31 15:21, Jens Axboe wrote:
>>> I also have another (maybe related?) question.  Apologies if this
>>> belongs in a separate thread, but are there any notes explaining why
>>> fio lays out the files before starting sequential writes?  The
>>> workload I was hoping to simulate is sustained, sequential writes to
>>> disk.  I'm trying to answer the question "How many simultaneous
>>> 200kBps writers can we support?"  Using my current jobs file, fio
>>> starts by creating the files (e.g "foo0: Laying out IO file(s) (1
>>> file(s) / 4MB)") before it starts processing.  However, creating the
>>> files in advance accounts for a chunk of performance that doesn't seem
>>> to be measured by fio.  Am I misunderstanding how to configure fio or
>>> its intended usage?
>>
>> You should be able to set overwrite=0 to avoid that. Are they random
>> writes?
>
> overwrite=0 is even default. I'm thinking the "Laying out IO file"
> message is confusing, it wont actually write contents first unless you
> ask it to (with eg overwrite=1).
>
>
> --
> Jens Axboe
>
>
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