From owner-freebsd-questions@freebsd.org Fri Sep 25 01:11:13 2015 Return-Path: Delivered-To: freebsd-questions@mailman.ysv.freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:1900:2254:206a::19:1]) by mailman.ysv.freebsd.org (Postfix) with ESMTP id 931C6A0925E for ; Fri, 25 Sep 2015 01:11:13 +0000 (UTC) (envelope-from paul@kraus-haus.org) Received: from mail-qg0-f48.google.com (mail-qg0-f48.google.com [209.85.192.48]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (Client CN "smtp.gmail.com", Issuer "Google Internet Authority G2" (verified OK)) by mx1.freebsd.org (Postfix) with ESMTPS id 5601D1FEA for ; Fri, 25 Sep 2015 01:11:13 +0000 (UTC) (envelope-from paul@kraus-haus.org) Received: by qgt47 with SMTP id 47so58978513qgt.2 for ; Thu, 24 Sep 2015 18:11:11 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20130820; h=x-gm-message-state:content-type:mime-version:subject:from :in-reply-to:date:content-transfer-encoding:message-id:references:to; bh=+JmkWvdbwXOYmkVTeaBSzylrfIujdPWbMm0wUPdPmGM=; b=J1JxQ52hGvQjBYBay87/9cayoXsZ0KStPObow4IDC1uVZPaoZL3BXPn2AawYWomr5w KZGZPqsjIBp27Fkqcm1YuDPPB6gBO7EtghGlSccq65uWDk5jBwpH4/Rzf+gGwYibdVWX 1O7KaxGrJL5dXFMr4YdvuZMGSONAxXX/0I4ZXVdS2lmk68mn9PyzQUj1BGNqSWjjXoLm Otc/Hy3mnfVPA/l3Xmk+kVQRtsshP+6l+kKv4txbKS6cFXXnWuymj/sl8sI7qbjFw/6Q w0f716fCCXKR5YmcmMMPUbVcr2JHbfBljFND07gvNGnr0sOek6AgiVW6PREN38DigD/G Exhg== X-Gm-Message-State: ALoCoQneW+70X3wU8JQXsN2QKD/7Yp01fUGtkcOhCVVDOLXfg8WJolNYacYY4u12qn7lEQw07+/c X-Received: by 10.140.39.168 with SMTP id v37mr3235453qgv.24.1443143471571; Thu, 24 Sep 2015 18:11:11 -0700 (PDT) Received: from [192.168.2.138] (pool-100-4-179-8.albyny.fios.verizon.net. [100.4.179.8]) by smtp.gmail.com with ESMTPSA id w3sm482814qha.0.2015.09.24.18.11.09 (version=TLSv1 cipher=ECDHE-RSA-RC4-SHA bits=128/128); Thu, 24 Sep 2015 18:11:09 -0700 (PDT) Content-Type: text/plain; charset=windows-1252 Mime-Version: 1.0 (Mac OS X Mail 7.3 \(1878.6\)) Subject: Re: sync vs async vs zfs From: Paul Kraus In-Reply-To: <560462C4.6030106@sneakertech.com> Date: Thu, 24 Sep 2015 21:11:07 -0400 Content-Transfer-Encoding: quoted-printable Message-Id: References: <56042774.6070404@sneakertech.com> <98BFE313-523F-4A2C-82BB-8683466068FB@kraus-haus.org> <560462C4.6030106@sneakertech.com> To: Quartz , FreeBSD questions X-Mailer: Apple Mail (2.1878.6) X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 25 Sep 2015 01:11:13 -0000 On Sep 24, 2015, at 16:53, Quartz wrote: >> Very short answer=85 >=20 > OK, thanks. So far that lines up with what I thought I knew. I still = think I might be fuzzy on what constitutes an 'app' in this context = though, presumably you're also counting services like nfs, etc? Anything that generates a FS read or write request :-) So yes, the = kernel NFS server counts. > Basically, when considering just boring file copies, which things are = or are not async and when? Under what circumstances is sync actually = used in the real world? I expect that system utilities like cp and tar do not do sync writes. = sync writes are supposed to be a special case, used only when needed. I = run into them with VBox writing to <>.vmdk files. > you can >> use a device which is much faster than the main zpool devices >=20 > Also >=20 > 1) A SLOG's only purpose is to reduce fragmentation and increase sync = speed, correct? Re: speed, using a SLOG that's the same speed as the = other drives in a pool is mostly pointless, right? Correct. And I proved that on one of my servers in pre-prodcuction = testing. I was able to find the bottleneck using iozone -o and then = added a mirrored pair of SSD as SLOG write performance went _down_ for = 4KB random writes! I then tested the SSDs on their own and confirmed = that the performance I was seeing was the native performance of the = SSDs. I asked for recommendations of a good, fast SSD over on the = OpenZFS list and ordered a pair of Intel 200 GB S3710 SSDs, they are = back ordered, so the server awaits full production use. =20 > 2) Async doesn't really care how your pool is constructed, and a SLOG = is really the only thing that seriously makes a difference for sync, = correct? Not quite true. Once you get through the ARC the configuration of the = zpool _will_ matter to performance. In fact, for reads, unless you = workload closely matches the prefetch algorithm, the zpool layout will = have an effect on performance. Remember, as a general rule, you get one = spindle=92s (drive=92s) worth of performance per top level vdev in the = zpool. So a zpool with one vdev that is an 8 drive RAIDz2 will have much = less performance than a set of 4 2-way mirrors. -- Paul Kraus paul@kraus-haus.org