On Nov 14, 2013, at 12:34 PM, Marcus Reid wrote:
> On Thu, Nov 14, 2013 at 06:35:43PM +0000, Teske, Devin wrote:
>> On Nov 14, 2013, at 10:21 AM, Allan Jude wrote:
>>> On 2013-11-14 9:34, Reid, Marcus wrote:
>>> I noticed a couple of things with the ZFS defaults that result from
>>> using the new installer in 10.0-BETA3.
>>> One, atime is turned off everywhere by default. There was a thread
>>> on this list on June 8 with a subject of 'Changing the default for
>>> ZFS atime to off?', and from what I can tell the idea of turning off
>>> atime by default was not a popular one.
>>> Two, and probably less controversial, is that fletcher4 is specified
>>> exlicitly on the root pool, even though it is default (wouldn't you
>>> just want to go with the default, in case it changes?)
>> I have never heard a good argument for having atime on.
> Can you address some of the issues that people brought up in the thread
> I mentioned earlier? I'll summarize some:
> - breaks some software (MTAs were mentioned), and the admin should
> know when to turn atime on in those cases.
> - "any mail program using mbox mail folders uses them to correctly
> report which mailboxes have not been read yet"
I'm looking at HEAD and I don't see "atime=off" for the /var dataset.
Knowing that most folks (accepting the defaults) will store their mail
in /var/mail ... does the concern over atime=off still exist?
However, I did notice that before we go creating the /var dataset, we
do set "atime=off" for the boot pool/dataset.
Is inheritance at-play here? and /var is turning up with atime=off even
though it's not specified?
In that case, I certainly agree we should remove atime=off from the last
place it is used -- the boot pool (nowhere else).
> - "Of course it can't be turned off by default. It is specified by
> - "If the overhead were a real problem then file systems would use a
> special atime cache"
>> The performance penalty on ZFS is quite large
> This is subjective, and if it were that bad it seems like other
> ZFS-based systems would have turned it off as well, or done something
> like caching/logging or relatime it to mitigate the performance hit.
I do recall discussions about bringing over relatime and making it the
Allan was the one that brought up the performance hit...
Similarly where I $work, we turn it off where need performance (large
storage arrays and such).
I gather it's only a matter of time until relatime is default.
>> and it also makes your snapshots grow constant.
> This does not seem to be the case. For example, if I snapshot
> zroot/usr/src and then tar it up, I generate a bunch of writes and the
> snapshot grows to 70.5MB. However, when I tar it up a second time,
> there are writes but the snapshot remains 70.5MB. It seems to use the
> same blocks for subsequent atime updates.
> If you made another snapshot, you would see more space used for that
> one, of course.
I'll defer to Allan to expand (was his recollection on snapshots)
>> If you have a use for it, you can turn it on I guess. This would be
>> solved by having the dataset editor we're planning for 10.1
>> Seems easy enough to turn on once installed. And like Allan says, the
>> editor for 10.1 would allow changing of the default.
> The same case could be made for turning it off, make that an option in
> the installer instead.
Fair enough. Good idea.
> Some people don't seem to use atime on a regular basis, but I use it a
> lot when troubleshooting things, and I think others do to when they get
> to the point where they realize how useful it is.
>> I'll add that if scripting it, you can (in current state for 10.0)
>> change the dataset options at-will.
>> The fletcher4 thing is a leftover, from older versions of ZFS where
>> the default was fletcher2, I suppose we can remove it
> While I'm picking nits:
> There are some other locally set things that are default (exec set to on
> on zroot/var/tmp is one), that might just inherit that from zroot/var.
Need a +1 from Allan on that.
> Also, zroot doesn't appear to be mounted on /zroot, as it is on other
> ZFS-based systems. This has the side-effect of newly created
> datasets not having a mountpoint by default, instead of /zroot/dataset.
> I don't know of the reason behind this change either.
Hmm, wonder if that's been fixed in HEAD already.
> I'm going to cross-post this to -current, this is a topic that I think
> warrants a wider audience.
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