From owner-freebsd-fs@freebsd.org  Fri Aug 10 14:39:51 2018
Return-Path: <owner-freebsd-fs@freebsd.org>
Delivered-To: freebsd-fs@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 D027D106C8F6
 for <freebsd-fs@mailman.ysv.freebsd.org>; Fri, 10 Aug 2018 14:39:51 +0000 (UTC)
 (envelope-from kostikbel@gmail.com)
Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1])
 (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 35EC28D6B7
 for <freebsd-fs@freebsd.org>; Fri, 10 Aug 2018 14:39:51 +0000 (UTC)
 (envelope-from kostikbel@gmail.com)
Received: from tom.home (kib@localhost [127.0.0.1])
 by kib.kiev.ua (8.15.2/8.15.2) with ESMTPS id w7AEdeCm041160
 (version=TLSv1.2 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=NO);
 Fri, 10 Aug 2018 17:39:43 +0300 (EEST)
 (envelope-from kostikbel@gmail.com)
DKIM-Filter: OpenDKIM Filter v2.10.3 kib.kiev.ua w7AEdeCm041160
Received: (from kostik@localhost)
 by tom.home (8.15.2/8.15.2/Submit) id w7AEdeS5041159;
 Fri, 10 Aug 2018 17:39:40 +0300 (EEST)
 (envelope-from kostikbel@gmail.com)
X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com
 using -f
Date: Fri, 10 Aug 2018 17:39:40 +0300
From: Konstantin Belousov <kostikbel@gmail.com>
To: Paul <devgs@ukr.net>
Cc: freebsd-fs@freebsd.org
Subject: Re: Question regarding relevance of syncer(4) in the context of ZFS
Message-ID: <20180810143940.GC2649@kib.kiev.ua>
References: <1533910747.525373107.k9z2n7hj@frv33.fwdcdn.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1533910747.525373107.k9z2n7hj@frv33.fwdcdn.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00,
 DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no
 autolearn_force=no version=3.4.1
X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home
X-BeenThere: freebsd-fs@freebsd.org
X-Mailman-Version: 2.1.27
Precedence: list
List-Id: Filesystems <freebsd-fs.freebsd.org>
List-Unsubscribe: <https://lists.freebsd.org/mailman/options/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-fs/>
List-Post: <mailto:freebsd-fs@freebsd.org>
List-Help: <mailto:freebsd-fs-request@freebsd.org?subject=help>
List-Subscribe: <https://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 10 Aug 2018 14:39:52 -0000

On Fri, Aug 10, 2018 at 05:28:11PM +0300, Paul wrote:
> Hello team,
> 
> 
> If my understanding is correct then ZFS does not need to be pushed around and being told when to sync data to devices.
> It is perfectly capable of keeping data consistent and synchronized according to configured options.
> 
> We even disable 'sync' option of file system that we use. But unfortunately we see a constant and periodical spikes 
> of load on our servers that are directly related to wake-ups of 'syncer' kernel daemon. Is it safe to assume that
> 'syncer' is not necessary on configurations that only use ZFS, and no other file systems? And hence, is it safe
> to assume that setting all of 'kern.filedelay', 'kern.dirdelay' and 'kern.metadelay' to some large values will not 
> compromise system's durability?

One of the job of the syncer is to convert dirty user-mapped pages into
the filesystem-specific write requests.  Without syncer touching the mount
point, corresponding pages could linger forever, or at least until a memory
pressure causes pagedaemon to clean them.