Light Intensity Modulation Direct Overwrite (LIMDOW) reduces the number of passes required to write on optical disks. What are the advantages of using LIMDOW and where is it useful?
LIMDOW media has four more layers than standard Magneto-Optical media. Each has very specific magnetic properties which change depending on temperature. Furthermore, laser power control in the drive is critical, since the temperature at the media recording layers determines the ultimate polarity of a magnetic transition. Low power is used for reading, medium for erasing and high power for writing. The various thin-film layers interact with each other depending on the current state of the written data (0 or 1) and what that state is being changed to. Manufacturing control of the recording layers and control of laser power are critical to successful recording. Although writing in one pass seems like an attractive idea, two immediate conclusions can be drawn.
First, the media has four additional layers per side, and therefore is more complicated to manufacture. LIMDOW media costs about 30 per cent more than standard MO media does. Second, the writing process is more complex and there is less margin for error. Recording margin is of use when media is recorded in one drive and read with another. There are also concerns about interchange between vendors of LIMDOW drives and media. In essence, a tradeoff of write performance versus media cost and reliability is made.
What are the advantages of LIMDOW?
LIMDOW offers increased write transfer rate over standard MO recording. The write transfer rate is double if there is no verify pass, or is improved by 50 per cent if the recommended verify pass is employed. What are the disadvantages? Customers who use MO technology are looking for highly reliable media that is readable in the long term. LIMDOW compromises recording margin and media compatibility while increasing media cost. There are only two vendors who currently supply LIMDOW media. LIMDOW recording is available for 4x drives, but will not initially be part of 8x drives.
For what specific applications would a user find LIMDOW capability especially useful?
LIMDOW recording is useful in applications where high write transfer rate is of critical importance. Backup is one such application. In this case the focus of the application is to write data quickly so that the system can be backed up within the backup window. Several backup software vendors have implemented a "pre-erase" process for MO disks. Pre-erase is a way of preparing the MO disk offline with an erase pass.
Then, the data is written in one pass during the backup. Since an additional verify pass is recommended in any case, direct over-write is of limited value in this application.
Are there any applications in which overwrite capability is not useful? Optical jukeboxes are most often used to provide nearline access to large amounts of reference data. This information is often generated by Document Image Management (DIM) or Computer Output to Laser Disk (COLD) systems. The process usually requires one or a few writes, and many reads. Therefore, the benefit of LIMDOW is minimised.
In Document Imaging applications, frequently accessed data is usually cached on a hard drive and the jukebox is used to provide random access to very large amounts of data. The time to swap media within the jukebox defines the access performance requirements, and usually outweighs the transfer rate performance of the drive. In many optical applications write-once media is used for regulatory or legal compliance reasons. There is no benefit to LIMDOW over MO in these situations, since write-once uses a single pass write.
At this time, Hewlett-Packard is not planning to offer LIMDOW drives in its product line. Concerns over the reliability of the technology, compatibility between vendors, and media cost outweigh the improvements in write performance. In the future, as the technology matures, Hewlett-Packard may choose to integrate it within optical jukeboxes.
( Hewlett-Packard 1999
Compiled by Rachel Hodgkins
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