From owner-freebsd-fs@freebsd.org Mon Sep 21 14:11:42 2015 Return-Path: Delivered-To: freebsd-fs@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 0EFF9A05111 for ; Mon, 21 Sep 2015 14:11:42 +0000 (UTC) (envelope-from kraduk@gmail.com) Received: from mail-wi0-x22a.google.com (mail-wi0-x22a.google.com [IPv6:2a00:1450:400c:c05::22a]) (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 9BF541973 for ; Mon, 21 Sep 2015 14:11:41 +0000 (UTC) (envelope-from kraduk@gmail.com) Received: by wiclk2 with SMTP id lk2so113655268wic.1 for ; Mon, 21 Sep 2015 07:11:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:in-reply-to:references:date:message-id:subject:from:to :cc:content-type; bh=EexZLTbFDwSHzh1X6fNeqOiVIuNG4qMHevlq/Ux4VI8=; b=rHrdpYQBY7ryOccwH3ji33Atw+W1MuwAnveJyOvTfz7qYTSuAUjkN1lyfLja5aU95R l//3ZrE9OyV0O/Bl3i657GnTikZ2qclkpDLvlyhZI3rSkDIXkkHAxyVT1aQYWgmQxzwH xX4sef+pfKjxBBFWDhr3JRpSSjf7harcTbNagFJCW2CmTmM61qcFAgv4D4nlNUENjNnE q+XnStig2Nm5fjpniMI8G/nBXw/u3Aa+fod7fWwU2RGeUzFL4avU9jdwrLI4coMh83AI x1TTffJwv0Hl+14nzosbMeS3oKBj6VODRGGP0/XhTvmXU0VjM2zepddrswwTrulZIJtc W4bw== MIME-Version: 1.0 X-Received: by 10.181.27.138 with SMTP id jg10mr14648827wid.29.1442844700012; Mon, 21 Sep 2015 07:11:40 -0700 (PDT) Received: by 10.28.125.18 with HTTP; Mon, 21 Sep 2015 07:11:39 -0700 (PDT) In-Reply-To: <56000CD8.4030208@sneakertech.com> References: <55FD9A2B.8060207@sneakertech.com> <56000CD8.4030208@sneakertech.com> Date: Mon, 21 Sep 2015 15:11:39 +0100 Message-ID: Subject: Re: ZFS cpu requirements, with/out compression and/or dedup From: krad To: Quartz Cc: FreeBSD FS Content-Type: text/plain; charset=UTF-8 X-Content-Filtered-By: Mailman/MimeDel 2.1.20 X-BeenThere: freebsd-fs@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Filesystems List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 21 Sep 2015 14:11:42 -0000 Nope DDT is only used for writes, zfs uses a free block space map, so only when a block is completely unreferenced will it be written to. The DDT is a table of blocks and their checksums. https://blogs.oracle.com/bonwick/en/entry/space_maps http://www.c0t0d0s0.org/archives/7271-ZFS-Dedup-Internals.html there are probably better references On 21 September 2015 at 14:57, Quartz wrote: > This is completely untrue, there performance issues with dedup are >> limited to writes only, as it needs to check the DDT table for every >> write to the file system with dedup enabled. Once the data is on the >> disk there is no overhead, and in many cases a performance boost as less >> data on the disk means less head movement and its also more likely to be >> in any available caches. If the write performance does become an issue >> you can turn it off on that particular file system. This may cause you >> to no longer have enough capacity on the pool, but then pools are easily >> extended. >> > > It still needs to keep the tables in memory as long as there's still > deduped data on disk though, right? Else what keeps track of which blocks > are used by which files? > > > _______________________________________________ > freebsd-fs@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-fs > To unsubscribe, send any mail to "freebsd-fs-unsubscribe@freebsd.org" >