Re: [PATCH V2] sd: Reduce log level to KERN_NOTICE for missing caching mode page

[Alain Kalker <a.c.kalker@gmail.com>]

On 06/02/2014 09:14 PM, James Bottomley wrote:
> On Mon, 2014-06-02 at 20:48 +0200, Alain Kalker wrote:
>> Reduce the lernel log level to KERN_NOTICE for messages related
>> to a missing caching mode page.
>>
>> Reasons why I think this change is justified:
>> - The condition is not an error; the existing workaround of assuming
>>    a write through cache doesn't limit functionality in any way.
>
> That's not actually correct.  The condition may be an error.  We have
> some USB attached storage devices with writeback caches which don't
> supply caching information.  Treating a writeback device as writethrough
> is a potential error because you may lose data as a result of it.

Aha, I thought that the write through treatment would always make sure 
that the device has written all the data before returning. If that's not 
always true, then I agree that the condition could be an error.

> The error is that we've made an assumption which may damage your data.
>
> The problem for us is that we can't tell if the assumption is going to
> be fatal or not ... the message is harmless for older USB devices.

I think that I wrongly assumed that only (older?) writethrough devices 
might not return caching information. If there are also writeback 
devices which don't return (correct) information, then I have to agree.

>> - It doesn't warrant a warning either: as it is most likely
>>    a hardware limitation, there is little that an ordinary user can do
>>    about it besides replacing the affected hardware.
>>    Making it a warning would still break quiet boot (kernel commandline
>>    parameter 'quiet'), and may still trip log monitoring software
>>    unneccessarily, either on every boot or whenever an affected
>>    external device is plugged in.
>> - It should be a notice, for it is a normal but significant condition,
>>    as it may impact write performance. This would bring it on par with
>>    the log messages in the same function.
>
> But this message may serve a purpose:  If you see it and you find you do
> have a USB device with a writeback cache, you can set the boot system to
> correct the cache type assumption and we'll send the correcting
> synchronize cache commands.

Very informative, thanks!
I do think that this should be documented somewhere, especially with 
regard to the risk of data loss. People might think that falling back on 
writethrough is always a 'safe bet' where it actually might not be.

> It certainly helps bug triagers identify issues which may require adding
> a particular device to the list that need to be treated as writeback.

Thanks for your feedback, much appreciated.

Alain