From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-fsdevel-owner@vger.kernel.org>
Received: from out1-smtp.messagingengine.com ([66.111.4.25]:40903 "EHLO
        out1-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK)
        by vger.kernel.org with ESMTP id S934880AbdIYT3o (ORCPT
        <rfc822;linux-fsdevel@vger.kernel.org>);
        Mon, 25 Sep 2017 15:29:44 -0400
From: Nikolaus Rath <Nikolaus@rath.org>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: Miklos Szeredi <mszeredi@redhat.com>,
        fuse-devel <fuse-devel@lists.sourceforge.net>,
        Maxim Patlasov <mpatlasov@virtuozzo.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [fuse-devel] [fuse] getattr() results ignored when writeback cache is active
References: <87zi9pk2ra.fsf@vostro.rath.org>
        <CAOssrKe_bEx_53JuRX_1tMw1mSy5vmNY=7BcMBD-4hx9A41uWQ@mail.gmail.com>
        <87vakdjsjj.fsf@vostro.rath.org>
        <CAOssrKeWuLfHE2W2_MGCgH6_j685p9xz_Uv9tQokDoz1a4vsYQ@mail.gmail.com>
        <87r2v1jphd.fsf@vostro.rath.org>
        <CAJfpegt7C-D6e7CGX_JOgO9Lw+unXPM7STmyjo8atw0grnZg5w@mail.gmail.com>
        <87tvzw9bma.fsf@vostro.rath.org>
        <20170925091218.GE19278@veci.piliscsaba.szeredi.hu>
Date: Mon, 25 Sep 2017 10:25:48 +0100
In-Reply-To: <20170925091218.GE19278@veci.piliscsaba.szeredi.hu> (Miklos
        Szeredi's message of "Mon, 25 Sep 2017 11:12:18 +0200")
Message-ID: <87lgl35dur.fsf@vostro.rath.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Sep 25 2017, Miklos Szeredi <miklos@szeredi.hu> wrote:
> Fuse supports the following I/O modes:
>
> - direct-io
> - cached
>   + write-through
>   + writeback-cache
>
> The direct-io mode can be selected with the FOPEN_DIRECT_IO flag in the
> FUSE_OPEN reply.
>
> In direct-io mode the page cache is completely bypassed for reads and wri=
tes.
> No read-ahead takes place. Shared mmap is disabled.
>
> In cached mode reads may be satisfied from the page cache, and data may be
> read-ahead by the kernel to fill the cache.  The cache is always kept con=
sistent
> after any writes to the file.  All mmap modes are supported.
>
> The cached mode has two sub modes controlling how writes are handled.  The
> write-through mode is the default and is supported on all kernels.  The
> writeback-cache mode may be selected by the FUSE_WRITEBACK_CACHE flag in =
the
> FUSE_INIT reply.
>
> In write-through mode each write is immediately sent to userspace as one =
or more
> WRITE requests, as well as updating any cached pages (and caching previou=
sly
> uncached, but fully written pages).  No READ requests are ever sent for w=
rites,
> so when an uncached page is partially written, the page is discarded.
>
> In writeback-cache mode (enabled by the FUSE_WRITEBACK_CACHE flag) writes=
 go to
> the cache only, which means that the write(2) syscall can often complete =
very
> fast.  Dirty pages are written back implicitly (background writeback or p=
age
> reclaim on memory pressure) or explicitly (invoked by close(2), fsync(2) =
and
> when the last ref to the file is being released on munmap(2)).  This mode
> assumes that all changes to the filesystem go through the FUSE kernel mod=
ule
> (size and atime/ctime/mtime attributes are kept up-to-date by the kernel)=
, so
> it's generally not suitable for network filesystems.  If a partial page is
> written, then the page needs to be first read from userspace.  This means=
, that
> even for files opened for O_WRONLY it is possible that READ requests will=
 be
> generated by the kernel.


Looks great to me.


Best,
-Nikolaus

--=20
GPG Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F

             =C2=BBTime flies like an arrow, fruit flies like a Banana.=C2=
=AB