linux-kernel - Re: submit bio directly from userspace

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Message-ID: <87si5kz2ta.fsf@openvz.org>
Date:	Fri, 09 Oct 2015 14:41:53 +0300
From:	Dmitry Monakhov <dmonakhov@...nvz.org>
To:	Mike Krinkin <krinkin.m.u@...il.com>, linux-kernel@...r.kernel.org
Cc:	axboe@...nel.dk, hch@....de, kmo@...erainc.com
Subject: Re: submit bio directly from userspace

Mike Krinkin <krinkin.m.u@...il.com> writes:

> Hi,
>

> i'm developing block layer workload simulator/player and need to
> generate bios as close as possible to the ones in original workload.
> To play workload i employ aio + O_DIRECT, but all IOs end up as bios
> with SYNC flag set. So i wonder is there any way to control bios
> flags from userspace or, maybe, any other options to generate
> workload from userspace except aio + O_DIRECT with more precise
> control over bio flags?
100% agree, I've investigated existing io-load replay tools and
seems that they all use O_DIRECT(like btreplay) or
buffered-aio/{sync_file_range,fadvise} ( like fio).
So it looks like they all very inaccurate.

Jens, what do to do to replay real load in order to test fs/ioscheduler
harness? Are you interested in having such fine-tuning interface
(probably guarded by CAP_SYS_ADMIN)?

There are several places where we can pass such flag
1) iocb->aio_flag, add IOCB_FLAG_DIRECT_ASYNC
2) add new flag O_DIRECT_ASYNC to file->f_flags

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