Re: Defaults in 10.0 ZFS through bsdinstall

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 ...

Regards, STefan