Recently, there has been a lot of discussion concerning reports of incompatibility between the Quantum Fireball drive and the PowerMacintosh (and PowerMac clones). Symptomatically, a drive installed inside some PowerMac's (or clones) will appear to develop recurring directory damage and file corruption. Typically, a drive which was working well will suddenly become corrupted on a restart or shutdown, or when the PowerMacintosh is brought out of its Energy-saving sleep mode.
This document is intended to dislcose 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 PowerMacintosh (or PowerMac clone) models.
On Friday 3/8/96, Quantum issued a "white paper" on potential compatibility issues when using Fireball drives internally with the PowerMacintosh; Apple, 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 incoporate a Write Cache. As such, this issue could theoretically 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:
1) Affected Fireball drives must be installed internally in a
PowerMacintosh or inside a PowerMac clone. The corruption
problems did NOT seem to occur on external drives, nor to
secondary internal Fireball drives that were installed along
with to an Apple-formatted primary drive.
2) The corruption problems occurred independently of the
formatting software used to prepare the Fireball drives.
Similar data corruption and directory damage was reported
on other Fireball drives purchased from virtually every
reseller or vendor of this drive model.
3) Even when installed internally as the only internal drive,
the corruption could not be reproduced repeatedly each time,
in every situation.
4) This corruption has been most prevalent with PowerMac
7100/66 Mac's, but has been seen occasionally with more
current PowerMac's. The corruption has never been observed
on drives installed in 68XXX Mac's, nor in PowerMac 6100's.
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 PowerMac'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 using utilities such as APS PowerControl and other third-party SCSI Mode Page editors, 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 PowerMac's, and is therefore a desirable feature.
When the PowerMac (or clone) 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 PowerMac 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 PowerMac shuts down, an external drive can finish flushing its RAM buffer and clear its cache completely, whether or not the PowerMac 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 behaviour 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-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, version 4.0.4 is the first formatting utility to incorporate these workarounds.
If you have existing Quantum Fireball hard drives, APS recommends the installation of this driver software.
