From owner-freebsd-questions@freebsd.org Thu Feb 14 15:47:34 2019 Return-Path: Delivered-To: freebsd-questions@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 543F214D89E8 for ; Thu, 14 Feb 2019 15:47:34 +0000 (UTC) (envelope-from ml@netfence.it) Received: from soth.netfence.it (net-2-44-121-52.cust.vodafonedsl.it [2.44.121.52]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "mailserver.netfence.it", Issuer "Let's Encrypt Authority X3" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id D5F7877350 for ; Thu, 14 Feb 2019 15:47:32 +0000 (UTC) (envelope-from ml@netfence.it) Received: from guardian.ventu (host42-227-static.19-80-b.business.telecomitalia.it [80.19.227.42]) (authenticated bits=0) by soth.netfence.it (8.15.2/8.15.2) with ESMTPSA id x1EFlLmJ001377 (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128 verify=NO) for ; Thu, 14 Feb 2019 16:47:23 +0100 (CET) (envelope-from ml@netfence.it) X-Authentication-Warning: soth.netfence.it: Host host42-227-static.19-80-b.business.telecomitalia.it [80.19.227.42] claimed to be guardian.ventu Subject: Re: ZFS and a spare SSD: ZIL or L2ARC? To: freebsd-questions@freebsd.org References: <20190213171142.33b7c430c5ec256b1cbafa42@sohara.org> From: Andrea Venturoli Message-ID: Date: Thu, 14 Feb 2019 16:47:22 +0100 User-Agent: Mozilla/5.0 (X11; FreeBSD amd64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.83 X-Rspamd-Queue-Id: D5F7877350 X-Spamd-Bar: ++++ Authentication-Results: mx1.freebsd.org X-Spamd-Result: default: False [4.79 / 15.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; FROM_HAS_DN(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; IP_SCORE(0.17)[ip: (0.58), ipnet: 2.44.0.0/16(0.29), asn: 30722(-0.07), country: IT(0.06)]; MIME_GOOD(-0.10)[text/plain]; PREVIOUSLY_DELIVERED(0.00)[freebsd-questions@freebsd.org]; HAS_XAW(0.00)[]; AUTH_NA(1.00)[]; RCPT_COUNT_ONE(0.00)[1]; NEURAL_SPAM_MEDIUM(0.87)[0.871,0]; MIME_TRACE(0.00)[0:+]; NEURAL_SPAM_SHORT(0.97)[0.968,0]; MX_GOOD(-0.01)[cached: mx.netfence.it]; NEURAL_SPAM_LONG(0.89)[0.891,0]; TO_DN_NONE(0.00)[]; R_SPF_NA(0.00)[]; DMARC_NA(0.00)[netfence.it]; FROM_EQ_ENVFROM(0.00)[]; R_DKIM_NA(0.00)[]; SUBJECT_ENDS_QUESTION(1.00)[]; ASN(0.00)[asn:30722, ipnet:2.44.0.0/16, country:IT]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2] X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 14 Feb 2019 15:47:34 -0000 On 2/14/19 3:48 PM, krad wrote: > You need to be careful with L2ARC as under certain conditions it can > actually degrade performance. This is down to you needing a data structure > in ram to manage the l2arc. This comes at the expense of ARC. However this > is more of an edge case when you are adding huge amounts of L2ARC compared > to system ram, but important in the enterprise world. This is interesting. I understand precise measurement would be required: are there proper tools to do this? Lacking them, are there "known cases" described? Like 16GiB RAM + 128 GiB SSD (which would be my case)? > From your described workload ZiL might be better suited as databases and > virtualisation often cause lots of sync writes. This is where Zil can help. > However the picture is never simple. If you are going to have a zil you > really want it mirrored This was also my impression: I would probably benefit more from a ZiL than an L2ARC, but as I said, I've only got a spare SSD (no mirror). Since this is more of a curiosity than a real need, I'm not risking data. bye & Thanks av.