Date: Fri, 15 Nov 2013 09:36:31 -0600 From: Mark Felder <feld@FreeBSD.org> To: freebsd-current@freebsd.org Subject: Re: Defaults in 10.0 ZFS through bsdinstall Message-ID: <1384529791.7937.47924713.3321BFEF@webmail.messagingengine.com> In-Reply-To: <5285E827.1090501@freebsd.org> References: <20131114173423.GA21761@blazingdot.com> <59A9B68B-4134-4217-83F3-B99759174EFE@fisglobal.com> <5285148E.6020903@allanjude.com> <3D3332FA-0ABF-4573-8E65-4E7FBB37100B@fisglobal.com> <1384462198.13183.47596065.6F8E7BCD@webmail.messagingengine.com> <55232624-3B76-4781-91E0-0C2A6260144D@fisglobal.com> <5285E827.1090501@freebsd.org>
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On Fri, Nov 15, 2013, at 3:23, Stefan Esser wrote: > Am 14.11.2013 22:02, schrieb Teske, Devin: > > On Nov 14, 2013, at 12:49 PM, Mark Felder wrote: > >> We don't even do installs on UFS with atime disabled by default in fstab > >> so why should we so suddenly change course for ZFS? > >> > > > > You've made a good point. > > There is major difference between UFS and ZFS: UFS allows in-place > updates of i-node fields (like atime), while ZFS uses COW for all > data, file contents and meta-data like the i-nodes. > > With atime ON on UFS you'll see a small number of writes on > file-systems that are only read - we are used to accept that. > > On ZFS every update of atime causes a write of the meta-data to > a free location on disk, then updates of all data structures > that reference that meta-data up to the root of the tree (the > uberblock). An update of a few bytes turns out to write tens > of KB for each atime update (within the TXG sync interval, which > defaults to 5 seconds on FreeBSD). If you create snapshots, then > each snapshot will contain a copy of the metadata that was valid > at the time of the snapshot (well, that's not so different from > the situation with UFS snapshots, just that the data structures > are much more complex and larger in the ZFS case). Due to the > ease and speed of snapshot creation with ZFS there probably are > a magnitude or more snapshots on a typical ZFS system than on > one using UFS (I currently have a few hundred and have turned off > periodic snapshot generation on many unimportant file-systems, > already). > > I really hope that we get relatime (with minor variations that > were discussed a few months ago) and that we make it the default > in some future release ... > Thanks for this in-depth explanation. I wasn't aware that atime was quite so expensive on ZFS.
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