 Quantum Fireball Drives and Power Macs
The following discussion was posted on Guy Kawasaki's Apple Developer List Server. The issue with Quantum Fireball hard drives has been resolved with DayStar's MP Floppy v2.6 (2 Disks). For more information, see the Quantum Drive Information Read Me. Recently, there has been a lot of traffic in this forum and in other newsgroups concerning reports of incompatibility between the Quantum Fireball drive and the Power Macintosh (and other Mac OS systems). This document is intended to disclose recent information and provide a clear solution path to users who have experienced data corruption and directory damage on the Fireball drive, when used internally with some Power Macintosh (or other Mac OS systems) models. On Friday 3/8/96, Quantum is expected to issue a "white paper" on potential compatibility issues when using Fireball drives internally with the Power Macintosh; sources inside Apple also report that they, too, will be releasing a similar white paper on this subject, probably within the next two to three weeks. Without going into the grisly details (which will be disclosed fully in the white papers), the Quantum Fireball drives are not solely the cause of the data corruption. A configuration concern has emerged that Apple has never had to address in the past, centering on drives that incorporate a Write Cache. As such, this issue can apply to almost any third-party hard drive that has been installed internally. This has been a difficult issue to isolate and identify because it has not been consistent. Both Quantum and Apple have been hard pressed to single out a definitive trigger. Similarly, APS (and other drive vendors) have been unable to demonstrate that there were any specific causes for the reports of repeated directory damage or file corruption. After gathering the facts for several months, a trend finally began to emerge:
After extensive testing and diagnostics, the cause of the problem was finally traced to the use of the Write Cache on these drives, and the fact that the Power Mac's in which they were installed were shutting down before the data in the drive's cache could be completely flushed to the media. Although the Write Cache can be turned off, users will observe a reduction in throughput. As the white papers will disclose, the use of write caching significantly improves any drive's performance on Power Mac's, and is therefore a desirable feature. When the Power Mac shuts down, it first clears its own System Cache by transferring the data to the drive. With Write Caching enabled, this data is transferred to a RAM buffer on the drive mechanism, which is then written to the drive's media. However, after the Power Mac has cleared it's own System Cache, it essentially thinks "Okay, I'm done," and powers itself down while the hard drive is in the process of transferring the data from its RAM buffer to the media. That's when the corruption occurs. If the drive is an external one, it has it's own power supply, so even when the Power Mac shuts down, an external drive can finish flushing its RAM buffer and clear its cache completely, whether or not the Power Mac itself is powered. This has never been an issue with Apple in the past because all of the drives they have previously used were shipped from the OEM drive makers with the Drive Cache turned off (at Apple's request). Only recently has Apple started using drives that have their Write Cache enabled. FYI - One of the drive models Apple is currently using happens to be the Quantum Fireball.] Presumably, Apple may have observed a similar behavior when they started using the Write Cache themselves because the current Apple drivers include a new command "SynchronizeCache", which is issued during shutdown. When issued, this command causes all data in the Apple&endash;formatted hard drive's cache to be completely flushed to the drive media before the Power Macintosh shuts down. Although the "SynchronizeCache" command is effective for many modern drives, some drives don't support it. In the white papers, alternate command methodologies have been recommended. Because APS has been closely involved in the confirmation and resolution of this issue, we will be among the first to incorporate these workarounds in our APS PowerTools formatting software, a version of which is currently being tested. The first official release of the new driver containing the implementations of these commands is expected be shipping by the middle of March '96, and will be designated as version 4.0.4. The current release is version 4.0.2; there will not be a 4.0.3 release. Other third-party formatting software is expected to follow suit.
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