Date: Wed, 14 Oct 1998 19:32:40 -0500 From: David Kelly <dkelly@hiwaay.net> To: freebsd-fs@FreeBSD.ORG, freebsd-scsi@FreeBSD.ORG Subject: Re: filesystem safety and SCSI disk write caching Message-ID: <199810150032.TAA12754@nospam.hiwaay.net> In-Reply-To: Message from Julian Elischer <julian@whistle.com> of "Wed, 14 Oct 1998 11:08:05 PDT." <Pine.BSF.3.95.981014105142.2872Q-100000@current1.whistle.com>
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Julian Elischer writes: > Hey everybody.. > look, we're in violent agreement here but haven't noticed it.... [...] > 3/ Given time and power, the layer that generates the completion reports > will sync down to the lower layers making them consistent. Lack of either > will have the lower layers in an inconsitent state. > > 4/ Drives that do not sync down after there is a scsi reset are in danger > of producing corrupted filesystems after a warm reboot. This should be > considered a firmware bug. This reminds me of PowerMac thing that cropped up a couple of years ago. MacUser and MacWorld published "comparison" tests of HD's and would happily rate Brand A's superior to Brand B's when both were the same HD simply because A's was faster in their benchmarks. Then people blindly bought the "superior" package over the other. So vendors shipped drives to maximize benchmark performance. Including enable of whatever write caching the drive could do internally. The problem that cropped up was that PowerMacs can turn themselves off. Including the internal HD. And on shutdown would turn themselves off so fast the HD lost unwritten cached data. About the last thing a Mac writes to a disk on shutdown is its clean flag. Users were being reminded on reboot that they were supposed to do a "Shutdown from the Special... menu" rather than yank the power. Eventually the driver writers (Apple's SCSI driver won't talk to non-Apple OEM'ed HD's) implemented a flush on final close. So the driver busy-waited until it thought the data was synced. If somebody is punching the CPU RESET, then there isn't much software can do to overcome deficiencies of firmware. Users with external HD's didn't have this "problem" because they had to turn off their HD's manually. -- David Kelly N4HHE, dkelly@nospam.hiwaay.net ===================================================================== The human mind ordinarily operates at only ten percent of its capacity -- the rest is overhead for the operating system. To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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