Date: Fri, 15 Nov 2013 18:15:08 +0200 From: Kimmo Paasiala <kpaasial@gmail.com> To: FreeBSD current <freebsd-current@freebsd.org> Subject: Re: Defaults in 10.0 ZFS through bsdinstall Message-ID: <CA%2B7WWScj3KZhFqgU=GLWvTDk5W7xcQLj4Nz5Y_g4%2B-7fzQrQYw@mail.gmail.com> In-Reply-To: <1384529791.7937.47924713.3321BFEF@webmail.messagingengine.com> 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> <1384529791.7937.47924713.3321BFEF@webmail.messagingengine.com>
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On Fri, Nov 15, 2013 at 5:36 PM, Mark Felder <feld@freebsd.org> wrote: > > > 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. What I did on my system when I was still using ZFS was that I set atime off by default but enabled it explicitly on /var/mail and /home datasets. The thought was that it's needed for mailboxes in /var/mail and if I then decide to move the inboxes to user's home directories I won't get any surprises. Would that be a suitable compromise here? -Kimmo
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