A key differentiator for products in the iSCSI storage industry are things like support for advanced features such as snapshots and replication. Snapshots of course encompass as single idea.. you tell the storage array to make a point in time copy of the state of the virtual disc. You can continue to read and write to the virtual disc but if you want, you can go back to the snapshot & see what things looked like at the moment that you captured it. Every iSCSI vendor products ship with this capability with the except of D-link DSN-3200-10 and Nexsan’s SATAbeast. But let me clarify.. they HAVE the capability, but they are simply tracking the delta between data versions. If you want to keep a snapshot forever, that process will take a lot of work to either copy it a new volume or from the parent volume. But a key differencitor between ALL of these iSCSI products is operating system and application support. It is not a good idea to have the storage system pick a moment in time to create the snapshot because the iSCSI storage protocal has no sense of files or directories. Depending on what is happening at that exact moment, the file system may or may not have been fully self consistent. There are a lot 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 email and databases where consistancy and a true or full backup may require multiple virtual discs to all be in sync at the same moment. If you plan to use snapshots of discs that are not in active use this is not a problem, however if you want to guarantee that a snapshot is fully self-consistent and can always be used in place of the original disc 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 the snapshot.
The leader in the agent support for snapshots is FalconStor’s NSS-S12 with specific support for more then a dozen database and email servers along with the most common Unix and Windows operation systems.
Replication is another data availability feature that you can use to differenciate between different I-SCSI people. Most often, replication is used to keep a copy of a virtual disk on another storage system with the idea that you would physically locate the second server in another building, location, county, whatever. Basic functionality is present in most products BUT there are many variations such as data de-duplication and bandwidth limiting capabilities.
With synchronous replication, every write to a virtual disc is mirrored to the replicated volume which means that the replicated volume is guaranteed to always be consistent with its original volume. It is called “synchronous” because the actual write operation is not signaled as completed until both copies are in sync. Synchronous replication is nuts because it requires high bandwidith and low latency between the two storage systems otherwise performance will be heavily impacted. Celeros, Compellent, FalconStor, LeftHand Networks, NetApp, Reldata and StoneFly all support synchronous replication but FalconStor offers the greatest options in supporting synchronous replication to both the same storage system and to a remote subsystem.
