From owner-freebsd-fs@FreeBSD.ORG  Fri Feb 20 12:53:02 2015
Return-Path: <owner-freebsd-fs@FreeBSD.ORG>
Delivered-To: freebsd-fs@freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org
 [IPv6:2001:1900:2254:206a::19:1])
 (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by hub.freebsd.org (Postfix) with ESMTPS id 15F3B979
 for <freebsd-fs@freebsd.org>; Fri, 20 Feb 2015 12:53:02 +0000 (UTC)
Received: from smtprelay01.ispgateway.de (smtprelay01.ispgateway.de
 [80.67.31.28])
 (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 C702F63E
 for <freebsd-fs@freebsd.org>; Fri, 20 Feb 2015 12:53:01 +0000 (UTC)
Received: from [78.35.156.159] (helo=fabiankeil.de)
 by smtprelay01.ispgateway.de with esmtpsa (TLSv1.2:AES128-GCM-SHA256:128)
 (Exim 4.84) (envelope-from <freebsd-listen@fabiankeil.de>)
 id 1YOn4f-0005d4-9S; Fri, 20 Feb 2015 13:52:53 +0100
Date: Fri, 20 Feb 2015 13:53:00 +0100
From: Fabian Keil <freebsd-listen@fabiankeil.de>
To: "Ilya A. Arkhipov" <rum1cro@yandex.ru>
Subject: Re: Disable zfs prefetch by default
Message-ID: <66c5011b.0fcc09f9@fabiankeil.de>
In-Reply-To: <54E71E20.8080303@yandex.ru>
References: <54E71E20.8080303@yandex.ru>
Reply-To: freebsd-fs@freebsd.org
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha1;
 boundary="Sig_/uyr9CYr8H=VxInaCrbfuAtz"; protocol="application/pgp-signature"
X-Df-Sender: Nzc1MDY3
Cc: freebsd-fs@freebsd.org
X-BeenThere: freebsd-fs@freebsd.org
X-Mailman-Version: 2.1.18-1
Precedence: list
List-Id: Filesystems <freebsd-fs.freebsd.org>
List-Unsubscribe: <http://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: <http://lists.freebsd.org/mailman/listinfo/freebsd-fs>,
 <mailto:freebsd-fs-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 20 Feb 2015 12:53:02 -0000

--Sig_/uyr9CYr8H=VxInaCrbfuAtz
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: quoted-printable

"Ilya A. Arkhipov" <rum1cro@yandex.ru> wrote:

> We have zfs prefetch enabled by default with > 4GB, but I didn't seen=20
> any case for using it, and all tuning guide saying to disable it.

Which tuning guides are you referring to?

> Maybe we disable zfs prefetch by default ? Or could you please provide=20
> why it needed.

Whether or not prefetching is useful depends on the workload
and the setup.

If you doubt that the prefetch default is useful for your workload
(for example because your systems mainly do small "random" reads
from "random" files), you can easily test this by disabling prefetching
for a while.

If this is too much work (or if no difference can be measured),
the default is probably fine.

Fabian

--Sig_/uyr9CYr8H=VxInaCrbfuAtz
Content-Type: application/pgp-signature
Content-Description: OpenPGP digital signature

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v2

iEYEARECAAYFAlTnLiYACgkQBYqIVf93VJ1lQACgyWFW051g5fIETiUbLJI5EgCX
wsUAn0bhu3VQHmU21DZchVoWcmIKDMle
=K0pp
-----END PGP SIGNATURE-----

--Sig_/uyr9CYr8H=VxInaCrbfuAtz--