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From: Cedric Blancher <cedric.blancher@gmail.com>
Date: Wed, 22 Nov 2023 23:47:17 +0100
Message-ID: <CALXu0UevOCU0dm-0WUEZZFCV=V8jQxmy2OQYhtptVyVAZeWs3g@mail.gmail.com>
Subject: Re: How does READ_PLUS differ from READ?
To: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
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On Sun, 19 Nov 2023 at 19:02, Anna Schumaker <schumaker.anna@gmail.com> wro=
te:
>
> 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>=
 wrote:
> > >
> > > 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.

How can the client figure out whether the data in a READ_PLUS reply
are zeros of data, or zeros from a hole?

Ced
--=20
Cedric Blancher <cedric.blancher@gmail.com>
[https://plus.google.com/u/0/+CedricBlancher/]
Institute Pasteur