[Bug 16490] "Assuming drive cache: write through" messages probably have wrong loglevel

bugzilla-daemon
2014-06-04T14:19:12Z

https://bugzilla.kernel.org/show_bug.cgi?id=16490

Alain Kalker <a.c.kalker@gmail.com> changed:

What |Removed |Added
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CC| |a.c.kalker@gmail.com

--- Comment #8 from Alain Kalker <a.c.kalker@gmail.com> ---
I submitted a patch to lower the log level to the linux-scsi mailing list.
The patch was not accepted; James Bottomley offered this very insightful
explanation why the log level should remain high (quoted here for those who are
not reading the linux-scsi mailing list):

> [...] 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.
>
> 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.

>From doing a little research, I would suggest these steps to mitigate the risk
(correct me if I'm wrong or have omitted anything):

- Find out if the device has write cache enabled by default, by consulting its
documentation, technical specifications, or by contacting the manufacturer.
- If the device does have write cache enabled, a quirk parameter can be added
to either the kernel commandline or as a module parameter for the usb-storage
module[1]
- Verify that the device works as expected.

[1] For more information on USB storage device quirks and how to configure
them, see linux/Documentation/kernel-parameters.txt in the Linux kernel source.

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