From owner-freebsd-current@freebsd.org  Sat Feb 10 18:46:38 2018
Return-Path: <owner-freebsd-current@freebsd.org>
Delivered-To: freebsd-current@mailman.ysv.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1])
 by mailman.ysv.freebsd.org (Postfix) with ESMTP id 2792FF133EB
 for <freebsd-current@mailman.ysv.freebsd.org>;
 Sat, 10 Feb 2018 18:46:38 +0000 (UTC)
 (envelope-from wollman@hergotha.csail.mit.edu)
Received: from hergotha.csail.mit.edu
 (wollman-1-pt.tunnel.tserv4.nyc4.ipv6.he.net [IPv6:2001:470:1f06:ccb::2])
 (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
 (Client did not present a certificate)
 by mx1.freebsd.org (Postfix) with ESMTPS id C4E1F6B9B2;
 Sat, 10 Feb 2018 18:46:37 +0000 (UTC)
 (envelope-from wollman@hergotha.csail.mit.edu)
Received: from hergotha.csail.mit.edu (localhost [127.0.0.1])
 by hergotha.csail.mit.edu (8.15.2/8.15.2) with ESMTP id w1AIkYbb000169;
 Sat, 10 Feb 2018 13:46:35 -0500 (EST)
 (envelope-from wollman@hergotha.csail.mit.edu)
Received: (from wollman@localhost)
 by hergotha.csail.mit.edu (8.15.2/8.14.4/Submit) id w1AIkX4Y000167;
 Sat, 10 Feb 2018 13:46:33 -0500 (EST) (envelope-from wollman)
Date: Sat, 10 Feb 2018 13:46:33 -0500 (EST)
From: Garrett Wollman <wollman@hergotha.csail.mit.edu>
Message-Id: <201802101846.w1AIkX4Y000167@hergotha.csail.mit.edu>
To: asomers@freebsd.org
Subject: Re: posix_fallocate on ZFS
X-Newsgroups: mit.lcs.mail.freebsd-current
References: <CAOtMX2jZr_kvJgOZWeiB-AZ3-7-uUu+UQ3P0nKhGZ0eNRzwMOQ@mail.gmail.com>
 <1e2f43fd-85da-6629-62d1-6e96790278e5@digiware.nl>
Organization: none
Cc: freebsd-current@freebsd.org
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2
 (hergotha.csail.mit.edu [127.0.0.1]); Sat, 10 Feb 2018 13:46:35 -0500 (EST)
X-Spam-Status: No, score=-1.0 required=5.0 tests=ALL_TRUSTED
 autolearn=disabled version=3.4.1
X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on
 hergotha.csail.mit.edu
X-Mailman-Approved-At: Sat, 10 Feb 2018 19:10:47 +0000
X-BeenThere: freebsd-current@freebsd.org
X-Mailman-Version: 2.1.25
Precedence: list
List-Id: Discussions about the use of FreeBSD-current
 <freebsd-current.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-current>, 
 <mailto:freebsd-current-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-current/>
List-Post: <mailto:freebsd-current@freebsd.org>
List-Help: <mailto:freebsd-current-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-current>, 
 <mailto:freebsd-current-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Sat, 10 Feb 2018 18:46:38 -0000

In article
<CAOtMX2jZr_kvJgOZWeiB-AZ3-7-uUu+UQ3P0nKhGZ0eNRzwMOQ@mail.gmail.com>,
asomers@freebsd.org writes:

>On Sat, Feb 10, 2018 at 10:28 AM, Willem Jan Withagen <wjw@digiware.nl>
>wrote:

>> Is there any expectation that this is going to fixed in any near future?

>No.  It's fundamentally impossible to support posix_fallocate on a COW
>filesystem like ZFS.  Ceph should be taught to ignore an EINVAL result,
>since the system call is merely advisory.

I don't think it's true that this is _fundamentally_ impossible.  What
the standard requires would in essence be a per-object refreservation.
ZFS supports refreservation, obviously, but not on a per-object basis.
Furthermore, there are mechanisms to preallocate blocks for things
like dumps.  So it *could* be done (as in, the concept is there), but
it may not be practical.  (And ultimately, there are ways in which the
administrator might manage the system that would defeat the desired
effect, but that's out of the standard's scope.)  Given the semantic
mismatch, though, I suspect it's unreasonable to expect anyone to
prioritize implementation of such a feature.

-GAWollman