Acorn Software's Virtual Branches

Coping with too much of a good thing

by Joel Snyder for Digital News and Review (ed: Chris Amaru), February 20, 1995, pp. 25-26.

CD-ROM media has become the standard for software distribution. With low cost and high reliability, CD-ROM software and documentation are available from every major software vendor. This brings with it another problem: managing all of those CD-ROMs. Digital's CONDIST (Consolidated Software Distribution) and OLD (Online Documentation) include almost 20 CD-ROMs; throw in the normal complement of 3rd party software providers and you could need two dozen CD-ROM drives just to keep everything at hand.

An inexpensive alternative for seldom-used CD-ROM media (aside from keeping them on a shelf) comes from Pioneer, which offers a digital data version of their popular automotive CD-ROM changer. The DRM-6000 series combines a single CD-ROM reader with a six-disk cartridge. For about twice the cost of a single CD-ROM reader, you get access to six CD-ROMs. More importantly, the DRM-6000 uses just one SCSI unit number. Because most VAX and Alpha systems have a single SCSI bus which can only accomodate seven SCSI units, the number of SCSI devices per system is strictly limited.

No special software is required to attach these changers to OpenVMS systems. The standard OpenVMS device driver will recognize and work with the CD-ROM changer, making each physical CD-ROM appear as a virtual device. We have used the Pioneer DRM-602x changer in our labs for several years to keep our six most-used CD-ROMs readily available. (The DRM-602x changer has a double-speed CD-ROM drive; the DRM-604x changer has a quad-speed drive)

Normally, multi-disk changers of this type include a "robot" which is responsible for changing disks. To control the robot, the operating system must issue special commands, which makes operation rather complex. The Pioneer drive simplifies this by automatically changing disks whenever a disk is accessed.

In a multi-user OpenVMS environment, this will cause severe performance problems and even system crashes on some hardware. As the OpenVMS device driver attempts to access more than one disk at the same time, the drive will begin to thrash, spending all of its time swapping disks and none of it reading data.

Solving the Thrashing Problem

Acorn Software's Virtual Branches makes the low-cost Pioneer changer work in a multi-user environment. By arbitrating between accesses to different disks, Virtual Branches keeps the disk drive from thrashing and the operating system from crashing.

Virtual Branches for the DRM-6000 series is a cut-down version of Acorn's optical storage library product, designed to handle disk changers with hundreds of CD-ROMs and complex robotic control systems. To start our test drive, we downloaded Version 1.2 of Virtual Branches from FTP.AcornSW.COM. The savesets are large (about 25 Mbytes) and include documentation on Virtual Branches in Microsoft Word format.

Because Acorn is aiming at the higher-end market, they haven't paid much attention to the smaller changers. Installation is very simple, using standard VMSINSTAL. Configuration is another story entirely. We found the manual incomprehensible and incomplete. Fortunately, we received excellent technical support (including two bug fixes) from the software developers via electronic mail and were able to get the software up in only a few days. Until Acorn redesigns their manuals, plan on having them walk you through the configuration. What should have been a fifteen minute job ended up taking much longer.

Once we got Virtual Branches up and running, it performed flawlessly in a single-system environment. We set the "dwell time" in Virtual Branches to be 30 seconds. Then, we set up multiple jobs on the same system attempting to access multiple disks in the changer. Virtual Branches correctly arbitrated among the jobs, giving each one its 30 seconds on the drive before swapping disks and letting another job have access to it.

When we tried to move into a VAXcluster configuration, things were not as stable. Acorn does not support the VAXcluster environment for the low-end DRM-6000 changers, although they are aware of the problem and plan to ship a working version soon.

In our performance tests using a DRM-602x (double-speed drive), we found no difference between using the changer with Virtual Branches and without. Virtual Branches uses a single system-resident daemon to control access to all of the optical libraries on a system. This consumes only minimal memory (50 Kbytes) and disk space (about 2 Mbytes).

Is it worth the aggravation?

Virtual Branches is far from a complete and polished product. It was hard to configure and didn't perform satisfactorily in a cluster environment. However, Acorn Software deserves the benefit of the doubt. As a software developer in the shrinking OpenVMS market, they're small enough to work with and were genuinely concerned (and responsive) when we couldn't get the software to work in our VMScluster. They've also got Glenn Everhart, well known as the premier non-Digital expert on disks and OpenVMS disk device drivers. The low-end market for CD-ROM changers is new to Acorn. If you need cluster access to a CD-ROM changer, it's worth talking to them about the current status of their product.

When used in conjunction with the Pioneer DRM-6000 CD-ROM changer, Virtual Branches solves a problem common at many Digital sites: too many CD-ROMs and not enough CD-ROM readers. By adding five of these changer CD-ROM readers to an older and slower VAXstation, such as a 4000/VLC or a 3100, you can build a network-accessible CD-ROM server with capacity for 30 CD-ROMs---for less than $4000. Adding the Virtual Branches software adds thousands of dollars to the price, but makes the server a network-wide resource, serving disks via DECnet or TCP/IP without worries.

Where to Buy

Pioneer DRM-602x ($450) and DRM-604x ($850 - $900) CD-ROM changers

Results! Computers and Video
Albuquerque, NM
+1 505 291 8998

Borealis Computer Systems
Aurora, CO
+1 800 536 8726
Virtual Branches (VAX or AXP) for DRM-6000 changers ($700), for DRM-18000
(18-disk) changers ($995) or for DRM-50000 (500-disk) changers ($4,795).
Virtual Branches bundled with a 6-disk changer ($2,445), 18-disk changer
($3,740), 500-disk changer ($23,790 to $26,790)
Acorn Software, Inc.
267 Cox St.
Hudson, Ma. 01749 USA
Office: (508) 568-1618
FAX: (508) 562-1133

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Last updated: March 3, 1995.