Using Windows Previous Versions to access ZFS Snapshots (July 14, 2009)

The Previous Versions tab on the Windows desktop provides a straightforward, intuitive way for users to view or recover files from ZFS snapshots.  ZFS snapshots are read-only, point-in-time instances of a ZFS dataset, based on the same copy-on-write transactional model used throughout ZFS.  ZFS snapshots can be used to recover deleted files or previous versions of files and they are space efficient because unchanged data is shared between the file system and its snapshots.  Snapshots are available locally via the .zfs/snapshot directory and remotely via Previous Versions on the Windows desktop.

Shadow Copies for Shared Folders was introduced with Windows Server 2003 but subsequently renamed to Previous Versions with the release of Windows Vista and Windows Server 2008.  Windows shadow copies, or snapshots, are based on the Volume Snapshot Service (VSS) and, as the [Shared Folders part of the] name implies, are accessible to clients via SMB shares, which is good news when using the Solaris CIFS Service.  And the nice thing is that no additional configuration is required - it "just works".

On Windows clients, snapshots are accessible via the Previous Versions tab in Windows Explorer using the Shadow Copy client, which is available by default on Windows XP SP2 and later.  For Windows 2000 and pre-SP2 Windows XP, the client software is available for download from Microsoft: Shadow Copies for Shared Folders Client.

Assuming that we already have a shared ZFS dataset, we can create ZFS snapshots and view them from a Windows client.

zfs snapshot tank/home/administrator@snap101
zfs snapshot tank/home/administrator@snap102

To view the snapshots on Windows, map the dataset on the client then right click on a folder or file and select Previous Versions.  Note that Windows will only display previous versions of objects that differ from the originals.  So you may have to modify files after creating a snapshot in order to see previous versions of those files.

The screenshot above shows various snapshots in the Previous Versions window, created at different times.  On the left panel, the .zfs folder is visible, illustrating that this is a ZFS share.  The .zfs setting can be toggled as desired, it makes no difference when using previous versions.  To make the .zfs folder visible:

zfs set snapdir=visible tank/home/administrator

To hide the .zfs folder:

zfs set snapdir=hidden tank/home/administrator

The following screenshot shows the Previous Versions panel when a file has been selected.  In this case the user is prompted to view, copy or restore the file from one of the available snapshots.


As can be seen from the screenshots above, the Previous Versions window doesn't display snapshot names: snapshots are listed by snapshot creation time, sorted in time order from most recent to oldest.  There's nothing we can do about this, it's the way that the interface works.  Perhaps one point of note, to avoid confusion, is that the ZFS snapshot creation time isnot the same as the root directory creation timestamp. In ZFS, all object attributes in the original dataset are preserved when a snapshot is taken, including the creation time of the root directory.  Thus the root directory creation timestamp is the time that the directory was created in the original dataset.

# ls -d% all /home/administrator
         timestamp: atime         Mar 19 15:40:23 2009
         timestamp: ctime         Mar 19 15:40:58 2009
         timestamp: mtime         Mar 19 15:40:58 2009
         timestamp: crtime         Mar 19 15:18:34 2009

# ls -d% all /home/administrator/.zfs/snapshot/snap101
         timestamp: atime         Mar 19 15:40:23 2009
         timestamp: ctime         Mar 19 15:40:58 2009
         timestamp: mtime         Mar 19 15:40:58 2009
         timestamp: crtime         Mar 19 15:18:34 2009

The snapshot creation time can be obtained using the zfs command as shown below.

# zfs get all tank/home/administrator@snap101
NAME                             PROPERTY  VALUE
tank/home/
administrator@snap101  type      snapshot
tank/home/
administrator@snap101  creation  Mon Mar 23 18:21 2009

In this example, the dataset was created on March 19th and the snapshot was created on March 23rd.

In conclusion, Shadow Copies for Shared Folders provides a straightforward way for users to view or recover files from ZFS snapshots.  The Windows desktop provides an easy to use, intuitive GUI and no configuration is required to use or access previous versions of files or folders.

REFERENCES FOR MORE INFORMATION

ZFS

ZFS Learning Center

Introduction to Shadow Copies of Shared Folders

Shadow Copies for Shared Folders Client

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