If there's any easy way to tell products apart in the iSCSI storage-area network server space, it's in the support for advanced data protection features such as snapshots and replication.

In theory, snapshots encompass a simple idea: you instruct the storage array to make a point-in-time copy of the state of a virtual disk. You can continue reading and writing to the virtual disk, but if you so desire, you can also go back to the snapshot to see what things looked like at the moment you captured it. That said, every vendor whose product ships with this capability (only D-Link's DSN-3200-10 and Nexsan's SATAbeast don't) takes great pains to document that their products aren't actually making a copy of the virtual disk, but rather, are simply tracking the delta between data versions. If you actually do want to keep a snapshot forever, that process may require some additional work to either copy it to a new volume or it from the parent volume.

One easy differentiator among the products tested regarding snapshots is operating system and application support. It is dangerous to have the storage system simply pick a moment in time to create the snapshot because the iSCSI storage protocol has no sense of files or directories. Depending on what is happening at that moment, the file system may or may not be fully self-consistent. There are lots of bits and pieces that point to each other on a disk, and if you catch a snapshot when one pointer is updated, but not the other, then the disk may not be 'legal' anymore. The problem compounds itself with certain applications, such as e-mail and databases, where consistency and a true or full backup may require multiple virtual disks to all be in synch at the same moment.

If you plan to make snapshots of disks that aren't in active use, then this will not be an issue. However, if you want to guarantee that a snapshot is fully self-consistent and can always be used in place of the original disk, you need to verify that your iSCSI SAN server has some sort of agent that can communicate between the operating system or application and the array to ensure consistency at the moment of snapshot.

The leader in claimed agent support for snapshots is FalconStor's NSS-S12, with specific support for more than a dozen databases and e-mail servers, along with the most common Unix and Windows operating systems. Unfortunately, our testing of FalconStor's snapshot capability with Windows 2008 showed a particularly pernicious bug in the product: it said that we were getting consistent snapshots, when we actually weren't. Eventually, technical support weighed in that Windows 2008 Server wasn't supported quite yet. While FalconStor's technology worked great with all of our other tests, this highlighted one of the difficulties of agent support.