Forgive my stupidity, but if I am depending on unlock_buffers() only
being called on a buffer after the buffer has reached disk (more
precisely, reached some state at which the disk won't lose it if power
fails), am I mistaken to depend on this, with the current SCSI drivers
and typical disk drives? This matters to me....
If I lose power, can I frequently lose a buffer write of a buffer that
has already passed through unlock_buffers?
This depends on the setting of Write Cache Enable in the mode pages on the disk
drive. If one cares about safety, the Write Cache should only be enabled if
the disks are connected to a UPS. As far as I know, no disk drive requires
more than a few seconds to update the platters from the cache, and it should do
so within a few seconds of becoming idle at worst. With a caching controller,
the controller should be configured for Write-Through unless the controller has
non-volatile storage that guarantees the cache will be written back in case of
power failure. A UPS is only sufficient if the controller will also guarantee
write-back in the case of a system reset.
Leonard
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