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Native Fibre Channel tape vs. Fibre Channel-to-SCSI converters.

More efficient tape backup/restore environments are one of the driving forces behind so many companies migrating to Fibre Channel SAN implementations. Until recently, tape drives and libraries with native Fibre Channel interfaces were rare. Backup administrators needed to use a Fibre Channel to parallel SCSI protocol converter (also called a Bridge or Router) to connect their native SCSI drives and libraries to the Fibre Channel SAN. Most major tape library vendors offered their products with built-in protocol converters, presenting a Fibre Channel-ready interface to the outside world.

Now that native Fibre Channel tape drives are readily available and are coming down in cost, these library manufacturers are offering products with a native Fibre Channel Tape solution in addition to the bridged solution. The article is intended to highlight the differences and benefits of both solutions to help administrators make an educated decision on which solution best fits their needs.

Upon taking a quick look, the decision appears to be simple. "Of course I want the library that comes with the native Fibre Channel tape drives. The protocol converter is just another piece of hardware to purchase, configure and manage. Not to mention the fact that it increases the overall risk of a system failure." In small, simple backup environments this is true. Where protocol converters begin to show value is in the larger (more than two tape drives in a library) more complex environments where performance, availability, connectivity, manageability and cost need to be considered more carefully. Here, Bridges and Routers offer more than simple protocol conversion. They add intelligence to the tape library and the backup/restore environment.

Connectivity

The fact that the majority of tape libraries still come equipped only with a SCSI-based robot for moving media presents a challenge for native Fibre Channel tape users. They require a separate SCSI host bus adapter (HBA) installed in the server to control the robot. This increases equipment and management costs, and occupies a PCI slot in the server. Utilizing a Fibre Channel-to-SCSI protocol converter inherently resolves this issue as the same SCSI busses on the converter can support both the tape drives and the robot.

Another consideration for larger libraries is that with Fibre Channel following a point-to-point connection topology, each tape drive requires a dedicated connection to the SAN. While switch ports are coming down in price, this can still become quite costly. Don't forget about the added cost of FC cables and optical transceivers when calculating the TCO for the native Fibre option.

The SCSI busses on the bridges can theoretically handle up to 15 drives per bus. In practice, only two to four drives are connected for performance reasons but the connectivity is still simplified. The aggregation of drives on the bridge can save money through higher utilization of switch ports.

Library manufacturers will be coming out with new products in the next year that will no longer require a SCSI connection to control the robot, but each tape drive will still require a dedicated Fibre Channel link to the SAN.

Performance

There is a common misconception that native Fibre Channel devices can outperform SCSI drives. The reality is that the type of external interface used by the drive does not change its ability to write or read data to tape. Tape performance is a function of how fast the drive can move data from its input buffers to the media, not the speed of the interface itself. For example, an LTO 3 drive is limited to approximately 80 MB/sec. uncompressed. Even though there is a 200 MB/sec. link feeding each drive, the performance is limited to what the drive itself can do. An Ultra320 SCSI bus on a bridge can easily sustain the performance to carry two or three LTO 3 drives.

If this is not convincing enough, SCSI LTO 3 drives have Ultra320 SCSI ports capable of 320 MB/sec. while native Fibre Channel LTO 3 drives have 2-Gb/sec. ports only capable of 200 MB/sec.

Regardless, the task of using a backup application to write a block of data to tape, even compressed, will not reach the maximum performance of the interface. Applications only use one outstanding command at a time and only send 64K of data in each block. Most bridges available today have speed enhancing features available that can actually increase performance over a standard Fibre connection to a native Fibre drive. The bridge will buffer up commands and data to effectively allow more than one command to be outstanding at any point in time. The result is that a host can issue new Write commands to a SCSI target in faster succession than is possible with standard architecture. The path between the host and the bridge is continuously filled with data, reducing the latency associated with pauses.

Availability

End users may tend to lean toward native Fibre Channel tape drives because they are dual ported which allows for the system design to include multiple redundant paths into the drives. The feeling is that this will increase the probability that unattended backups will complete overnight.

Most Fibre-to-SCSI converters offer redundant paths on the Fibre Channel side as they have multiple independent ports and built-in failover control. Each SCSI tape drive can be mapped to both Fibre ports providing two independent paths for each drive. Should an error occur with one of the active ports, data can be automatically re-routed to the standby port. This provides additional data redundancy and improved disaster recovery for backup systems.

The challenge with the bridge option is that SCSI tape drives are single ported. This leaves you open to bridge or SCSI link failures, but the failure rate of these devices compared to the tape drives, media, cables, optical transceivers and everything else in the configuration is relatively low. Remember that even a dual ported native FC drive is still only a single drive.

Manageability

Native Fibre drives do not provide very comprehensive management capabilities. In fact, most allow only limited capabilities in-band over the Fibre Channel link. Specialized management software packages need to be purchased to query the status of the drives. Since each drive is connected directly to the SAN, administrators need to manage each drive independently, again increasing management costs.

The intelligence the bridge products add to managing the backup environment is one of the most significant benefits over native Fibre tape. The bridge products allow for consolidated management of all drives and the robot in one place using browser-based remote monitoring and management GUIs. These tools are typically supplied with every bridge at no additional cost and includes such features as SNMP management capabilities, email/phone home capabilities, advanced event and error logging with battery backed time stamp recording, link up/down monitoring, SCSI drive health monitoring, temperature and voltage monitoring and many more easy-to-use configuration capabilities.

In addition, some bridges allow for in-field troubleshooting and diagnostics via a built-in Fibre Channel and SCSI bus trace log that provides bus analyzer-like capabilities. A key total cost of ownership factor to consider is how long it takes to identify and resolve an issue once a hardware failure occurs. The troubleshooting assist capabilities provided in the bridge products can reduce down time considerably. They can also communicate in band over the Fibre Channel link to provide monitoring and management for customers who already have Fibre Channel SAN Management tools in place.

In addition to the advanced management and performance enhancing capabilities, most bridges also include features to keep the backup process alive upon unforeseen circumstances. For example, they have the ability to respond to an inquiry from a host in proxy of a drive that may be busy. This comes into play when multiple hosts are controlling backups through the same library. If host #1 is backing up to a tape drive while host #2 tries to query a drive, the drive will not respond because it is busy with the backup. This typically results in the backup application on host #2 getting confused. The result is either the tape drive is not detected or the application hangs.

Serverless Backup is another value add feature. Bridges support the Extended Copy SCSI-3 command while most native Fibre drives do not. This allows backups to occur without passing data through the server. It acts as the data mover, resulting in backups completing quicker without utilizing valuable CPU cycles.

Cost

System Administrators need to consider the entire cost of the solution when comparing native Fibre tape to a Fibre Channel-to-SCSI protocol converter. On average, a native Fibre Channel tape drive costs around $500 more than the same drive with a parallel SCSI interface. Each native drive requires a dedicated Fibre Channel switch port. At approximately $1000 per port this can add up quickly. The need for additional Fibre Channel cables, optical transceivers and a separate SCSI card to control the robot also results in a considerably higher native Fibre tape solution cost.

One final consideration: If all the tape library manufacturers offer their products with native Fibre Channel LTO 3 drives, how will they distinguish theirs against the competitor? As SANs emerge as the leading method to transition from a server-based to a storage-based architecture, customers need to consider special-function products. Adding intelligence is the key to adding value to the network. Even as native Fibre Channel tape drives gain popularity, it is likely that library manufacturers will continue to offer products that include bridging options with SCSI drives.

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Publication:Computer Technology Review
Date:Mar 1, 2006
Words:1579
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