From owner-freebsd-fs@FreeBSD.ORG Thu May 12 10:03:36 2011 Return-Path: Delivered-To: freebsd-fs@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 5CDD7106564A for ; Thu, 12 May 2011 10:03:36 +0000 (UTC) (envelope-from alexander@leidinger.net) Received: from mail.ebusiness-leidinger.de (mail.ebusiness-leidinger.de [217.11.53.44]) by mx1.freebsd.org (Postfix) with ESMTP id CCF488FC16 for ; Thu, 12 May 2011 10:03:35 +0000 (UTC) Received: from outgoing.leidinger.net (p5B155ED8.dip.t-dialin.net [91.21.94.216]) by mail.ebusiness-leidinger.de (Postfix) with ESMTPSA id E2840844015; Thu, 12 May 2011 12:03:20 +0200 (CEST) Received: from webmail.leidinger.net (webmail.Leidinger.net [IPv6:fd73:10c7:2053:1::2:102]) by outgoing.leidinger.net (Postfix) with ESMTP id 0421A128C; Thu, 12 May 2011 12:03:18 +0200 (CEST) Received: (from www@localhost) by webmail.leidinger.net (8.14.4/8.13.8/Submit) id p4CA3HOg038348; Thu, 12 May 2011 12:03:17 +0200 (CEST) (envelope-from Alexander@Leidinger.net) Received: from pslux.ec.europa.eu (pslux.ec.europa.eu [158.169.9.14]) by webmail.leidinger.net (Horde Framework) with HTTP; Thu, 12 May 2011 12:03:17 +0200 Message-ID: <20110512120317.12543g51m4im15k4@webmail.leidinger.net> Date: Thu, 12 May 2011 12:03:17 +0200 From: Alexander Leidinger To: =?utf-8?b?wqlpbXVu?= Mikecin References: <4DCA5620.1030203@dannysplace.net> <4DCB455C.4020805@dannysplace.net> <4DCB81B8.6070301@digsys.bg> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; DelSp="Yes"; format="flowed" Content-Disposition: inline Content-Transfer-Encoding: quoted-printable User-Agent: Dynamic Internet Messaging Program (DIMP) H3 (1.1.6) X-EBL-MailScanner-Information: Please contact the ISP for more information X-EBL-MailScanner-ID: E2840844015.ADE71 X-EBL-MailScanner: Found to be clean X-EBL-MailScanner-SpamCheck: not spam, spamhaus-ZEN, SpamAssassin (not cached, score=0, required 6, autolearn=disabled) X-EBL-MailScanner-From: alexander@leidinger.net X-EBL-MailScanner-Watermark: 1305799403.3855@Z7UYgEbaHiH2ZjllWstZjA X-EBL-Spam-Status: No Cc: freebsd-fs Subject: Re: ZFS: How to enable cache and logs. X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 12 May 2011 10:03:36 -0000 Quoting =C2=A9imun Mikecin (from Thu, 12 May 2011 =20 10:02:42 +0200): > > On 12. svi. 2011., at 08:44, Daniel Kalchev wrote: > >> On 12.05.11 05:26, Danny Carroll wrote: >>> >>> - Don't use SSD for the Log device. Write speed tends to be a problem= . >> It all depends on your usage. You need to experiment, unfortunately. > > What is the alternative for log devices if you are not using SSD? > Rotating hard drives? > > AFAIK, two factors define the speed of log device: write transfer =20 > rate and write latency. There is also bus contention (either on the SCSI bus, or in the SATA =20 channel/controller, or on the PCI-whatever (e/X/y) bus). > You will not find a rotating hard drive that has a write latency =20 > anything near the write latency of even a slowest SSD you can find =20 > on the market. > On the other hand, only a very few rotating hard drives have a write =20 > transfer rate that can be compared to SSD's. And if your PCI-something bus is not saturated but your SCSI/SATA =20 controller struggles with the work which is thrown at it, a separate =20 log device (normal HD) on another controller could free up the =20 pool-controller(s) up to a situation where it can handle all requests =20 at full speed and the log-controller can provide the additional =20 throughput at full speed which the pool-controller was not able to =20 satisfy. What you do in this case is that you add more spindles (disks) =20 dedicated to sync-write operations. The normal RAID-common-knowledge =20 of "adding more spindles for more performance" applies here, just that =20 it is specially for sync-write operations. The generic hint to have =20 them faster than the pool-disks is an answer for the worst case. As =20 always, the worst case for one person may not be the worst case for =20 another persons workload. Bye, Alexander. --=20 If love is the answer, could you rephrase the question? =09=09-- Lily Tomlin http://www.Leidinger.net Alexander @ Leidinger.net: PGP ID =3D B0063FE7 http://www.FreeBSD.org netchild @ FreeBSD.org : PGP ID =3D 72077137