On Thu, Apr 15, 2004 at 08:33:14AM -0500, Greg Miller wrote:support that. The FreeBSD folks tried to solve this by turning off write caching by default. Unfortunately, this hurt performance so much they had to turn it back on and just recommend SCSI drives for important data.
Why, do SCSI drives all come with battery-backed cache? (So when you power them up again they complete the old cached write.) I didn't think so, but would be pleased to learn otherwise...
No, but the OS can ensure that the ordering constraints are honored on any writes that actually make it to the disk. That's the only constraint the OS makes anyway, since it ensures that the only disk corruption that can occur is that some disk space that is currently unused may still appear to be in use. Then when the system boots after a failure, the system snapshots the disk, and fsck runs in the background to free up that unused space in the background. That's how FreeBSD avoids journalling.
Of course, with a good UPS *AND* the proper software running to react to signals from the UPS, you get that sort of protection for free, and you certainly want the system UPS anyway. But that's also much more complicated and vulnerable to failures due to misconfigured software, so it'd sure be nice to have the hard-drive-UPS as well.
I suspect there's just not enough demand. People that need the safety just go out and by SCSI drives, IDE drives with tagged queueing, or a general purpose UPS.
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