DVD as a viable medium for traditional Back-up and Archiving of Mission Critical Data

With the advent of DVD rewriteable technology a viable alternative to traditional back-up and archiving now exists. From the P.C. on the desktop to the enterprise server it is now possible to use a medium that promises to be faster and more cost effective than traditional digital tape storage.

While DVD-RAM technology is finally here, it is the future technology path that will place DVD-RAM storage as the primary choice for backup and archiving.

The first generation DVD drives will read and write at 1.4 MB/s, which considering the infancy of the technology is excellent. The second generation of DVD drives have been announced with an approximate doubling of the numbers with a capacity of 9.4GB (2 x 4.7 GB) and read and write rates of 2.8 MB/sec. DVD-RAM will eventually get to a 100 GB platter (using both sides) and we can expect a comparable increase in performance in transfer rates along with this increase in capacity.

As IT organizations look for more responsive and cost effective solutions for data storage the appeal of DVD is very strong. However there are some pitfalls that must be avoided.

From a storage library point of view besides the capacity and speed of a DVD drive the DVD technology strong points are the low cost of the drive mechanisms themselves and the fact that the drive is “mechanically “ interchangeable with a CD-ROM or CD-R drive. These are also the weak points when we apply the technology to mission critical data storage.

Low cost commercial drives are typically not as robust as industrial  tape drives particularly when employed in high usage robotics environment. The major weakness is usually in the mechanical loading system, which is not designed to withstand the duty cycle of a robotics library. The DVD drives do not normally report their status and performance for a system to monitor as when used for they’re intended purpose on the desktop there is no need to provide this function.  However when the DVD drive is used in a robotics library for industrial use it becomes very important for the system administrator to be aware of potential problems with the back-up and archiving hardware.  The second disadvantage comes from the fact that it is a relatively simple task to replace a CD-ROM drive in a CD Jukebox with a DVD-RAM mechanism. The danger here is that the low cost CD Jukebox was designed to simply load disks, which being “read only” or in the case of CD-R “write once” to copy from a master file. Error recovery routines and high data availability are not fundamental requirements for CD ROM jukeboxes. However in the traditional back up and archiving tape storage world they are mandatory.

These issues can however with careful design be overcome, making the DVD-RAM technology a viable method of performing mission critical backup.

The ASACA Teracart^® DVD –RAM libraries utilize specially designed carriers that allow the library to track performance and usage of the drive mechanism itself. This data is available via ASACA’s ROBOWEB control panel and provides the administrator with performance data and wear-out data of the drive mechanisms. The carriers used in the library allow the end user to easily replace drives that the system has monitored and detected problems either with loading and unloading the media or with abnormal spin up times to come ready.  The Teracart libraries also monitor power on hours and load unload cycles and provide the system administrator with vital information to allow system preventative maintenance to be performed when necessary.

In the event that a drive needs to be replaced the Teracart library can take the malfunctioning drive off line from the host SCSI bus and allow the drive to be replaced (the new drive will automatically assume the SCSI ID of the old drive) without disrupting the operation of the remaining drives in the library, or any other SCSI devices connected to the Host computer.

The robotics control for a mission critical data library need to be able to recover from all manner of problems and situations. Use of complex retry and error correcting algorithms is necessary to provide reliable access to data. As mechanical systems wear the critical positioning to consistently load and unload drives and media cartridges must be maintained.  The high end machines used for archiving and back-up currently in use around the world will have self learning robotic mechanisms that will, in the event of a load or unload failure execute a retry algorithm that repositions the robotics in small increments in all of the axes until a successful load or unload has been completed. Generally the robotics software will learn this “new position” as the preferred loading point for that particular location.  Many of the lower cost CD jukeboxes do not have this level of sophistication that is typically found on Tape and Magneto-Optical libraries.

The ASACA DVD library can learn and adapt offset positions through out the media storage and drives as the system usage increases. In addition the use of a double picker mechanism for the media handler allows for redundant operation in case of a malfunction

Finally there is the need to keep track of the location of data disks. It is vital that the relative location of all disks be maintained at all times. In the event of a power failure the library must be able to recover, determine the location of all media (in the drives or handler) and return the media to it’s proper location in order to continue operation as normal. The ASACA DVD-RAM libraries will complete a full error recovery routine to ensure that the disk location is accurate at all times.

If proper design considerations are given to the robotics for DVD media it is possible to provide a high performance low cost storage units that utilizes commercially available DVD RAM drives and media.

The ASACA AM750DVD library provides 3.9TB of storage in a footprint of less than 4 square feet and with a capacity of up to 12 DVD-RAM drives and 750 disks provides sufficient storage and throughput to handle the majority of storage needs for most businesses.