Re: Some very basic questions

[jim owens <jowens@hp.com>]

Avi Kivity wrote:
> Tejun Heo wrote:
>> For most SATA drives, disabling write back cache seems to take high
>> toll on write throughput.  :-(
> 
> I measured this yesterday.  This is true for pure write workloads; for 
> mixed read/write workloads the throughput decrease is negligible.

Different tests on different hardware
give different results at different times!

>> As long as the error status is sticky, it doesn't have to hold on to
>> the data, it's not gonna be able to write it anyway.  The drive has to
>> hold onto the failure information only.  Yeah, but fully agreed on
>> that it's most likely dependent on the specific firmware.  There isn't
>> any requirement on how to handle write back failure in the ATA spec.
>> It wouldn't be too surprising if there are some drives which happily
>> report the old data after silent write failure followed by flush and
>> power loss at the right timing.
> 
> I got flamed for this on another list, but let's disable the write cache 
> and live with the performance drop.

We don't get to decide this, customers do.
As they say in the raid forum... fast, cheap, good - pick any 2

We just need to ensure we don't turn good into bad with fs mistakes.

jim

P.S. no flames because we chose no-battery == disable-write-cache