Date: Thu, 1 Mar 2012 14:32:33 +0000 From: Attilio Rao <attilio@freebsd.org> To: Pawel Jakub Dawidek <pjd@freebsd.org> Cc: Konstantin Belousov <kostikbel@gmail.com>, arch@freebsd.org, Gleb Kurtsou <gleb.kurtsou@gmail.com> Subject: Re: Prefaulting for i/o buffers Message-ID: <CAJ-FndCSPHLGqkeTC6qiitap_zjgLki%2B8HWta-UxReVvntA9=g@mail.gmail.com> In-Reply-To: <20120301141247.GE1336@garage.freebsd.pl> References: <20120203193719.GB3283@deviant.kiev.zoral.com.ua> <CAJ-FndABi21GfcCRTZizCPc_Mnxm1EY271BiXcYt9SD_zXFpXw@mail.gmail.com> <20120225151334.GH1344@garage.freebsd.pl> <CAJ-FndBBKHrpB1MNJTXx8gkFXR2d-O6k5-HJeOAyv2DznpN-QQ@mail.gmail.com> <20120225194630.GI1344@garage.freebsd.pl> <20120301111624.GB30991@reks> <20120301141247.GE1336@garage.freebsd.pl>
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2012/3/1, Pawel Jakub Dawidek <pjd@freebsd.org>: > 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. Thanks, Attilio -- Peace can only be achieved by understanding - A. Einstein
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