On Fri, Nov 15, 2013 at 5:36 PM, Mark Felder <feld_at_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? -KimmoReceived on Fri Nov 15 2013 - 15:15:09 UTC
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