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.Received on Fri Nov 15 2013 - 14:36:34 UTC
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