Eric Schrock's Blog

Month: September 2006

I’ve been meaning to get around to blogging about these features that I
putback a while ago, but have been caught up in a few too many things.
In any case, the following new ZFS features were putback to build 48 of
Nevada, and should be availble in the next Solaris Express

Create Time Properties

An old RFE has been to provide a way to specify properties at create
time. For users, this simplifies admnistration by reducing the number
of commands which need to be run. It also allows some race conditions
to be eliminated. For example, if you want to create a new dataset with
a mountpoint of ‘none’, you first have to create it and the underlying
inherited mountpoint, only to remove it later by invoking ‘zfs set

From an implementation perspective, this allows us to unify our
implementation of the ‘volsize’ and ‘volblocksize’ properties, and pave
the way for future create-time only properties. Instead of having a
separate ioctl() to create a volume and passing in the two size
parameters, we simply pass them down as create-time options.

The end result is pretty straightforward:

# zfs create -o compression=on tank/home
# zfs create -o mountpoint=/export -o atime=off tank/export

‘canmount’ property

The ‘canmount’ property allows you create a ZFS dataset that serves
solely as a mechanism for inheriting properties. When we first created the
hierarchical dataset model, we had the notion of ‘containers’ –
filesystems with no associated data. Only these datasets could contain
other datasets, and you had to make the decision at create-time.

This turned out to be a bad idea for a number of reasons. It
complicated the CLI, forced the user to make a create-time decision that
could not be changed, and led to confusion when files were accidentally
created on the underlying filesystem. So we made every filesystem able
to have child filesystems, and all seemed well.

However, there is power in having a dataset that exists in the hierarchy
but has no associated filesystem data (or effectively none by preventing
from being mounted). One can do this today by setting the ‘mountpoint’
property to ‘none’. However, this property is inherited by child
datasets, and the administrator cannot leverage the power of inherited
mountpoints. In particular, some users have expressed desire to have
two sets of directories, belonging to different ZFS parents (or even to
UFS filesystems), share the same inherited directory. With the new
‘canmount’ property, this becomes trivial:

# zfs create -o mountpoint=/export -o canmount=off tank/accounting
# zfs create -o mountpoint=/export -o canmount=off tank/engineering
# zfs create tank/accounting/bob
# zfs create tank/engineering/anne

Now, both anne and bob have directories at ‘/export/’, except that
they are inheriting ZFS properties from different datasets in the
hierarchy. The adminsitrator may decide to turn compression on for one
group of people or another, or set a quota to limit the amount of space
consumed by the group. Or simply have a way to view the total amount of
space consumed by each group without resorting to scripted du(1).

User Defined Properties

The last major RFE in this wad added the ability to set arbitrary
properties on ZFS datasets. This provides a way for administrators to
annotate their own filesystems, as well as ISVs to layer intelligent
software without having to modify the ZFS code to introduce a new

A user-defined property name is one which contains a colon (:). This
provides a unique namespace which is guaranteed to not overlap with
native ZFS properties. The emphasis is to use the colon to separate a
module and property name, where ‘module’ should be a reverse DNS name.
For example, a theoretical Sun backup product might do:

# zfs set com.sun.sunbackup:frequency=1hr tank/home

The property value is an arbitrary string, and no additional validation
is done on it. These values are always inherited. A local adminstrator
might do:

# zfs set localhost:backedup=9/19/06 tank/home
# zfs list -o name,localhost:backedup
tank            -
tank/home       9/19/06
tank/ws         9/10/06

The hope is that this will serve as a basis for some innovative products
and home grown solutions which interact with ZFS datasets in a
well-defined manner.

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