Boot environments and /var - what should be shared/unshared?

[Trond Endrestøl]

On Tue, 17 Jan 2017 19:46-0600, Brandon J. Wandersee wrote:

> Arthur Chance writes:
> 
> > I've been using beadm boot environments for a while now, but still
> > haven't seen a definitive reference for which parts of /var should be
> > shared across boot environments and which should be per environment. Is
> > there such a document, or is everybody still busking it?
> >
> > Some top level directories in /var seem fairly obviously in need of
> > sharing, others less so, and /var/db seems to be a horrible mix of stuff
> > that could be shared across releases and stuff that might break horribly
> > if shared over major revisions. (That's "might" because I have no idea
> > if it would in practice.)
> >
> > I doubt whether any of this would matter (except for space) if one
> > simply rolled forward monotonically, but a roll back because of
> > problems, or simply switching between development environments, could
> > lose necessary updates if the directory hadn't been shared when it
> > should have been.
> 
> I suppose I can't give you a "definitive reference," but I selected
> things to omit from boot environments based on advice in the book
> /FreeBSD Mastery: Advanced ZFS" by Michael W. Lucas and Alan
> Jude. Here's what I've got, follow by the reasoning behind it:
> 
> |  NAME                     USED  AVAIL  REFER  MOUNTPOINT
> |  workbox/VARIABLE        3.82M  72.5G   144K  none
> |  workbox/VARIABLE/crash   144K  72.5G   144K  /var/crash
> |  workbox/VARIABLE/log    2.86M   497M  1.80M  /var/log
> |  workbox/VARIABLE/mail    340K  72.5G   196K  /var/mail
> |  workbox/VARIABLE/tmp     360K  72.5G   144K  /var/tmp
> 
> 
> 1. /var/crash, /var/tmp: The contents of these directories are supposed
>                          to store non-volatile data, some of which is
>                          relevant to recovery. You don't want it
>                          disappearing after a reboot, or want to have to
>                          find and mount an extra ZFS dataset in order to
>                          access it.
>    
> 2. /var/mail, /var/log: Mail and logs are generated by applications as
>                         well as the base system, and are
>                         time-sensitive. You don't want these skipping
>                         around through time at all, but it's more
>                         egregious if you lose a day's worth of logs for
>                         your web server because something in the base
>                         system got messed up. That's exactly the sort of
>                         thing the BSD filesystem hierarchy was designed
>                         to prevent in the first place.
> 
> 
> I've also got a couple datasets mounted in /var, but as children of
> another tree:
> 
> |  workbox/APPS           6.26G  72.5G  5.42G  /usr/local
> |  workbox/APPS/config    7.17M  72.5G  6.22M  /usr/local/etc
> |  workbox/APPS/pkgCache   238M  2.77G   236M  /var/cache/pkg

> |  workbox/APPS/pkgDB      162M  72.5G  81.5M  /var/db/pkg

If you decide to use base as a set of packages, as proposed for 
11.1-RELEASE, I would reconsider having /var/db/pkg and the /usr/local 
subtree as separate datasets/filesystems. It might be better to treat 
base and 3rd party packages as a whole, due to 
/var/db/pkg/local.sqlite recording everything installed in the system.

> This way I can roll back package upgrades all at once without touching
> anything else on the system. I used to handle packages/ports within boot
> environments, but decided to separate them once I realized that doing so
> could make them grow pretty large with successive upgrades. Having your
> ports/packages tied to boot environments is really only useful when
> tracking -CURRENT or performing a major version upgrade, so I see it as
> a waste to constantly take up space in preparation for something that
> happens once every couple years, for which I'll have ample warning and
> preparation.
> 
> Someone else might come along with better ideas, but I suspect a lot of
> it depends on taste and personal need.

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