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From:   Anna Schumaker <schumaker.anna@gmail.com>
Date:   Sun, 19 Nov 2023 13:02:19 -0500
Message-ID: <CAFX2JfnDC1xZvuuUiHe8_RpBehwCZriZ13yYPk6pQSPSV4V-qQ@mail.gmail.com>
Subject: Re: How does READ_PLUS differ from READ?
To:     Cedric Blancher <cedric.blancher@gmail.com>
Cc:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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On Sun, Nov 19, 2023 at 12:59=E2=80=AFPM Cedric Blancher
<cedric.blancher@gmail.com> wrote:
>
> On Sun, 19 Nov 2023 at 18:48, Anna Schumaker <schumaker.anna@gmail.com> w=
rote:
> >
> > Hi,
> >
> > On Sun, Nov 19, 2023 at 12:38=E2=80=AFPM Cedric Blancher
> > <cedric.blancher@gmail.com> wrote:
> > >
> > > Good evening!
> > >
> > > How does READ_PLUS differ from READ? Has anyone made a simpler
> > > presentation (PowerPoint slides) than the RFCs?
> >
> > No slides, but at a high level READ_PLUS can compress out long ranges
> > of zeroes in a read reply by returning a HOLE segment instead of the
> > actual zeroes. It's perfectly valid for the server to skip the zero
> > detection and return everything as a data segment, however.
>
> So how do you differ between
> 1. a hole, aka no filesystem blocks allocated
> 2. a long sequence of valid data with all zero bytes in them

That's up to the server! It could use something like fiemap or lseek
with SEEK_HOLE or SEEK_DATA. It could also scan the data to see if
there are any zeroes that could be compressed out.

Anna

>
> Ced
> --
> Cedric Blancher <cedric.blancher@gmail.com>
> [https://plus.google.com/u/0/+CedricBlancher/]
> Institute Pasteur