From owner-freebsd-arch@FreeBSD.ORG Thu Mar 1 15:11:18 2012 Return-Path: Delivered-To: arch@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 2AB101065672; Thu, 1 Mar 2012 15:11:18 +0000 (UTC) (envelope-from asmrookie@gmail.com) Received: from mail-ee0-f54.google.com (mail-ee0-f54.google.com [74.125.83.54]) by mx1.freebsd.org (Postfix) with ESMTP id 7A0C48FC12; Thu, 1 Mar 2012 15:11:17 +0000 (UTC) Received: by eekd17 with SMTP id d17so253248eek.13 for ; Thu, 01 Mar 2012 07:11:16 -0800 (PST) Received-SPF: pass (google.com: domain of asmrookie@gmail.com designates 10.112.9.34 as permitted sender) client-ip=10.112.9.34; Authentication-Results: mr.google.com; spf=pass (google.com: domain of asmrookie@gmail.com designates 10.112.9.34 as permitted sender) smtp.mail=asmrookie@gmail.com; dkim=pass header.i=asmrookie@gmail.com Received: from mr.google.com ([10.112.9.34]) by 10.112.9.34 with SMTP id w2mr2545454lba.50.1330614676462 (num_hops = 1); Thu, 01 Mar 2012 07:11:16 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=gamma; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:cc:content-type; bh=7uAs/tmWbzC6/wr3qSuwtfYxQxRiocUfZf7/D4d583M=; b=BQ49iAOcE/voiKi4gCH8gIidDjjfXohLxk18U0k4SxxjijmjSDB1qTzgjFFIq+1MBC b77zOxS78xIGJPwQzxnHjYfzMbU44c9YV184oT0gHVgWreEIEyGf+4ovm/cLrBXQAmQ/ 9T/sPv6rmOqufpBYJebSAsoh/J6skE2t08SVQ= MIME-Version: 1.0 Received: by 10.112.9.34 with SMTP id w2mr2071242lba.50.1330614676363; Thu, 01 Mar 2012 07:11:16 -0800 (PST) Sender: asmrookie@gmail.com Received: by 10.112.41.5 with HTTP; Thu, 1 Mar 2012 07:11:16 -0800 (PST) In-Reply-To: <20120301150125.GX55074@deviant.kiev.zoral.com.ua> References: <20120203193719.GB3283@deviant.kiev.zoral.com.ua> <20120225151334.GH1344@garage.freebsd.pl> <20120225194630.GI1344@garage.freebsd.pl> <20120301111624.GB30991@reks> <20120301141247.GE1336@garage.freebsd.pl> <20120301144708.GV55074@deviant.kiev.zoral.com.ua> <20120301150125.GX55074@deviant.kiev.zoral.com.ua> Date: Thu, 1 Mar 2012 15:11:16 +0000 X-Google-Sender-Auth: gJ-0HKxicl_VFuRPeeNGI95A3gU Message-ID: From: Attilio Rao To: Konstantin Belousov Content-Type: text/plain; charset=UTF-8 Cc: arch@freebsd.org, Gleb Kurtsou , Pawel Jakub Dawidek Subject: Re: Prefaulting for i/o buffers X-BeenThere: freebsd-arch@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Discussion related to FreeBSD architecture List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 01 Mar 2012 15:11:18 -0000 2012/3/1, Konstantin Belousov : > On Thu, Mar 01, 2012 at 02:50:40PM +0000, Attilio Rao wrote: >> 2012/3/1, Konstantin Belousov : >> > On Thu, Mar 01, 2012 at 02:32:33PM +0000, Attilio Rao wrote: >> >> 2012/3/1, Pawel Jakub Dawidek : >> >> > On Thu, Mar 01, 2012 at 01:16:24PM +0200, Gleb Kurtsou wrote: >> >> >> On (25/02/2012 20:46), Pawel Jakub Dawidek wrote: >> >> >> > - "Every file system needs cache. Let's make it general, so that >> >> >> > all >> >> >> > file >> >> >> > systems can use it!" Well, for VFS each file system is a >> >> >> > separate >> >> >> > entity, which is not the case for ZFS. ZFS can cache one block >> >> >> > only >> >> >> > once that is used by one file system, 10 clones and 100 >> >> >> > snapshots, >> >> >> > which all are separate mount points from VFS perspective. >> >> >> > The same block would be cached 111 times by the buffer cache. >> >> >> >> >> >> Hmm. But this one is optional. Use vop_cachedlookup (or call >> >> >> cache_entry() on your own), add a number of cache_prune calls. It's >> >> >> pretty much library-like design you describe below. >> >> > >> >> > Yes, namecache is already library-like, but I was talking about the >> >> > buffer cache. I managed to bypass it eventually with suggestions from >> >> > ups@, but for a long time I was sure it isn't at all possible. >> >> >> >> Can you please clarify on this as I really don't understand what you >> >> mean? >> >> >> >> > >> >> >> Everybody agrees that VFS needs more care. But there haven't been >> >> >> much >> >> >> of concrete suggestions or at least there is no VFS TODO list. >> >> > >> >> > Everybody agrees on that, true, but we disagree on the direction we >> >> > should move our VFS, ie. make it more light-weight vs. more >> >> > heavy-weight. >> >> >> >> All I'm saying (and Gleb too) is that I don't see any benefit in >> >> replicating all the vnodes lifecycle at the inode level and in the >> >> filesystem specific implementation. >> >> I don't see a semplification in the work to do, I don't think this is >> >> going to be simpler for a single specific filesystem (without >> >> mentioning the legacy support, which means re-implement inode handling >> >> for every filesystem we have now), we just loose generality. >> >> >> >> if you want a good example of a VFS primitive that was really >> >> UFS-centric and it was mistakenly made generic is vn_start_write() and >> >> sibillings. I guess it was introduced just to cater UFS snapshot >> >> creation and then it poisoned other consumers. >> > >> > vn_start_write() has nothing to do with filesystem code at all. >> > It is purely VFS layer operation, which shall not be called from fs >> > code at all. vn_start_secondary_write() is sometimes useful for the >> > filesystem itself. >> > >> > Suspension (not snapshotting) is very useful and allows to avoid some >> > nasty issues with unmounts, remounts or guaranteed syncing of the >> > filesystem. The fact that only UFS utilizes this functionality just >> > shows that other filesystem implementors do not care about this >> > correctness, or that other filesystems are not maintained. >> >> I'm sure that when I looked into it only UFS suspension was being >> touched by it and it was introduced back in the days when snapshotting >> was sanitized. >> >> So what are the races it is supposed to fix and other filesystems >> don't care about? > > You cannot reliably sync the filesystem when other writers are active. > So, for instance, loop over vnodes fsyncing them in unmount code can never > terminate. The same is true for remounts rw->ro. > > One of the possible solution there is to suspend writers. If unmount is > successfull, writer will get a failure from vn_start_write() call, while > it will proceed normal if unmount is terminated or not started at all. I don't think we implement that right now, IIRC, but it is an interesting idea. Attilio -- Peace can only be achieved by understanding - A. Einstein