On 03/01/2011 02:33 PM, Markus Trippelsdorf wrote:
FUA support is currently switched off by default in drivers/ata/libata-core.c. Given that many modern drives do support FUA now, wouldn't it make sense to switch it on without setting a (undocumented) kernel/module parameter?
I believe I proposed this some time ago. Essentially all modern drives should support FUA now, since it's part of the definition of the NCQ (FPDMA) read/write commands. However, as I recall one of the objections to enabling it was that since it's just a bit in a command, there's a possibility that some drives may ignore it by accident or design, which is less likely with an explicit cache flush command. I'm not very inclined to agree myself (if you go down that road of pre-emptively predicting drive implementer stupidity, where do you stop?) but that's what was raised.
Another complication is that NCQ can be disabled at runtime either by user request or by error-handling fallback, and not all drives that support NCQ also support the FUA versions of the non-NCQ read/write commands, so changes in NCQ enable status may also need to result in changes in FUA support status on the block device.
I believe the way the block layer uses it, basically it only saves the overhead of one transaction to the drive. It might be significant on some workloads (especially on high IOPS drives like SSDs) but it's likely not a huge deal.
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