December 1998
Juke Packs 8X MO Drives
Lowell Rapaport
When Sony introduced the new 5.2 gigabyte (GB) magneto optical drives earlier this year, it created a flurry of activity among jukebox vendors updating their jukeboxes. Overnight, the capacity doubled. Higher-capacity drives and discs spin faster and take more time to spin up. Jukebox firmware was updated to take the longer disk mounting times into account.
The Hewlett Packard 400ex is a mid-sized sibling in a family of jukeboxes using the same internal robotics. The 400ex has 76 media slots (for a total of nearly 400 GB of storage) and two magneto-optical drives. Similar to the 400ex are the 320ex, which is the same jukebox with 64 media slots and four drives, and the 160ex, which is half the size of the 320ex with 32 media slots and two drives. The 400ex is 36 inches wide, 19 inches deep and 34 inches tall; a little smaller than a floor-standing office copier.
The magneto optical drives contained within the 400ex are the latest technology 5.2 GB (sometimes referred to as 8X) drives. The drives will read and write 5.2-GB and 2.6-GB non-LIMDOW media and will read 650 megabyte and 1.3 gigabyte disks. The read speed of the Sony drives is rated at about 5 megabytes per second. Write speed with write verification turned on should be about 1.5 megabytes per second. I confirmed these specifications by moving large blocks of data to and from the jukebox to measure sustained transfer rates. When properly set up with jukebox management software that caches input and output, most users should experience performance close to that of a networked hard drive.
Since the drives are non-LIMDOW, they take about two or three times longer to record than to read, depending on whether write verification is turned on or not. Hewlett Packard rates the write speed at 2.3 megabytes per second, but this is without write verification. Write verification should always be turned on for extra reliability. Data is written to a disk only once, but is read repeatedly. If a block of data is written incorrectly, the loss of data can potentially be disastrous.
Disk load times, as tested, were between five and six seconds for the longest distance disk move in the jukebox, from the mail slot to the drives. This agrees with HP's 5.5 second rating and includes the drive's spin up time. Unloading a disk took just 3 to 4 seconds, including spin down time. The rated disk eject time is 3 seconds. The jukebox has a "dual picker" that can hold two disks at once. This speeds up swap times and eliminates the need to keep one slot free or to use the mail slot for swapping disks. The swap time is the sum of the times it takes to unmount one disk from a drive and mount a second disk. This worked out to about 8 or 9 seconds.
With two drives and assuming a short queue of requests for disks in the jukebox, time to data should be no more than 8 to 12 seconds. A well-designed caching scheme should be able to further shorten the time to data for most users, especially in high input/output volume environments.
Of interest to system administrators is the connection characteristics of the 400ex. The unit has a flexible SCSI port. It is daisy chainable, and you can choose which kind of SCSI connection to make, single-ended or differential SCSI-2 in both 16-bit wide and 8-bit narrow cable widths. Wide SCSI-2's 20-megabyte-per-second speed is faster than the 400ex will ever provide data, but it allows for up to 16 devices on the SCSI chain. The 400ex has three separate SCSI ID numbers, one for each drive and one for the jukebox robotics.
Although cable length is not critical in differential SCSI mode, it is for single-ended SCSI. Since a large jukebox like the ex400 can't always be located near your server, Hewlett Packard has thoughtfully designed the SCSI port to act as a repeater, extending the length and improving the quality of the SCSI signal. This helps reduce SCSI problems and errors.
Hewlett Packard rates the jukebox mean time between failure (MTBF) at 100,000 hours. At 24 hours-per-day operation, this should give you a life span of more than ten years. HP also rates the robotics and the drives at 2 million and 750,000 swaps, respectively. At that rate, the jukebox would have to perform more than 400 disk swaps per day to severely tax the drives and robotics.
The 400ex jukebox is a heavy-duty piece of equipment and is fast and reliable enough to use in any high-demand storage environment.
--by Lowell Rapaport
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