From: Sitsofe Wheeler <sitsofe@gmail.com>
To: smitha sunder <sundersmitha@gmail.com>
Cc: "Elliott, Robert (Servers)" <elliott@hpe.com>, fio <fio@vger.kernel.org>
Subject: Re: unaligned IO and starting LBA
Date: Thu, 19 Dec 2019 03:27:47 +0000 [thread overview]
Message-ID: <CALjAwxhuNDowJbesAjgPVoj67h5tfLj-OAcWdA0go30Qy95+Gg@mail.gmail.com> (raw)
In-Reply-To: <CA+zhgFRJJ6JiWe46+UPDkfDddUDxCTJcOTPUtJnA-y6rKcvN6w@mail.gmail.com>
On Thu, 17 Oct 2019 at 13:49, smitha sunder <sundersmitha@gmail.com> wrote:
>
> Hi Robert,
>
> Since I want to use blocksize_unaligned=1 , I had to get rid of direct=1 :
>
> Per man page :
> blocksize_unaligned, bs_unaligned
> If set, any size in blocksize_range may be used. This typically won't
> work with direct I/O, as that normally requires sector alignment.
>
> # fio --name=job1 --filename=/dev/nvme0n1 --rw=randwrite
> --ioengine=libaio --number_ios=1 --debug=io --bs_unaligned=1
> --bs=1029 --direct=1
> fio: set debug option io
> fio: bs_unaligned may not work with raw io
> On Wed, Oct 16, 2019 at 8:12 AM Elliott, Robert (Servers)
> <elliott@hpe.com> wrote:
> >
> > > -----Original Message-----
> > > From: fio-owner@vger.kernel.org [mailto:fio-owner@vger.kernel.org] On Behalf Of smitha sunder
> > > Sent: Tuesday, October 15, 2019 4:37 PM
> > > To: fio <fio@vger.kernel.org>
> > > Subject: unaligned IO and starting LBA
> > >
> > > Hello,
> > >
> > > I have a 512 byte formatted nvme drive and I want to send an unaligned
> > > IO and here’s the fio command line I am using , fio version fio-3.7
> > >
> > > # fio --name=job1 --filename=/dev/nvme0n1 --rw=randwrite
> > > --ioengine=libaio --number_ios=1 --debug=io --bs_unaligned=1
> > > --bs=1029
A blocksize of 1029 is neither a multiple of 4096 nor 512! You simply
can't send I/O that is not aligned (and is not a multiple) of the
device's logical block size down to a device so something will have to
"fix" things for you.
> > > Shouldn’t I see D7BD7CE as the starting LBA for the write based on the
> > > above offset (0x1af7af9cec) ?
> >
> > Without direct=1 you're exercising the OS page cache logic,
> > not directly accessing the drive.
Robert's right - when you have buffering going on the linux kernel
will quietly do a read-modify-write (two reads assuming it didn't
already have those blocks in memory, a modify, then two writes)
because you straddled a block.
--
Sitsofe | http://sucs.org/~sits/
prev parent reply other threads:[~2019-12-19 3:28 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-15 21:36 unaligned IO and starting LBA smitha sunder
2019-10-16 15:11 ` Elliott, Robert (Servers)
2019-10-16 17:18 ` smitha sunder
2019-12-19 3:27 ` Sitsofe Wheeler [this message]
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