Drive Snapshot offers the possibility to use the Windows build in Volume Snapshot Copy Service (VSS) for the creation of consistent images.
Main advantage compared to the internal Drive Snapshot Driver is the
ability to image a running MS Exchange server, and also to image several
partitions at the same time.
(See below for technical Details).
By using VSS, several drives can be imaged at the same time:
from the GUI by holding the Control key while clicking in the list
from the command line with
c:>snapshot c:+d:+e: x:\backup\computer\$DISK-FULL.sna
if saving several disks at the same time, the destination MUST contain $DISK somewhere to differentiate the different image files
c:>snapshot c:+d:+e: x:\computer\$DISK-$DATE-Diff.sna -hx:\backup\computer\$DISK-FULL.hsh
works only from the command line
snapshot hd1:* x:\backup\$disk.sna
saves all partitions of physical drive 1, incl.
Linux, Dell, Compaq,...
Since Windows XP Windows implements a 'Volume ShadowCopy
Service' (VSS), that essentially provides the same service as the build in
Snapshot driver - that is a consistent snapshot of one (or more) partitions at a
fixed point in time.
Additionally, it defines and implements a broadcast similar to:
To everybody who cares about VSS: would you please go into a consistent state, bring transactions to a recoverable state, etc., and than please be write for some (short) time because I'm going to make a backup.
At this time, the current state of the disk is frozen, and made
available for backup programs until.
Changes to the drive while the backup is active are written to some reserved space on the disk, until the backup finishes and tells VSS to release the buffers.
Can snapshot several volumes at the same time
By using above mentioned broadcast it can also put Exchange
Server 2003 into a stable state without shutting down exchange services,
which isn't always reliable possible without using VSS.
For other databases this isn't necessary (they seem to be implemented in a more stable fashion), but at least using VSS doesn't hurt.
by the fixed (preconfigured) size of the VSS buffers, at
least in theory this buffer can overflow, and thus abort the backup.
the build in driver writes changes immediately to the image, and thus can't overflow.
by the additional buffering on disk, using VSS may be slower then the internal driver.
VSS doesn't exist for Windows NT and Windows 2000.
VSS simply doesn't work, if some older products (like older version of MS SQL) are running on more recent windows versions; NTBACKUP doesn't work in these cases either.
Implementation of VSS vs.
As both drivers have their pros and cons, by default the VSS driver is only used if
MS Exchange 2003 or Active Directory are detected
a backup of several volumes is started
else the internal driver is used.
You may change this by using the [Advanced Options] dialog, or from command line by
--noVSS: never uses VSS
--UseVSS: always uses VSS, if it exists and works
--ForceVSS: tries to use VSS; if VSS is not available for some reason, backup is aborted with error
vssadmin list writersSnapshot can use VSS in two different modes.