Re: bcache and hibernation

[Nikolaus Rath <Nikolaus@rath.org>]

Hi Michael,

On Apr 05 2018, Michael Lyle <mlyle@lyle.org> wrote:
> On 04/05/2018 01:51 AM, Nikolaus Rath wrote:
>> Is there a way to prevent this from happening? Could eg the kernel
>> detect that the swap devices is (indirectly) on bcache and refuse to
>> hibernate? Or is there a way to do a "true" read-only mount of a
>> bcache volume so that one can safely resume from it?
>
> I think you're correct.  If you're using bcache in writeback mode, it is
> not safe to hibernate there, because some of the blocks involved in the
> resume can end up in cache (and dependency issues, like you mention).

Could you explain why this isn't a problem with writethrough? It seems
to me that the trouble happens when the hibernation image is *read*, so
why does it matter what kind of write caching is used?

> I am unaware of a mechanism to prohibit this in the kernel-- to say that
> a given type of block provider can't be involved in a resume operation.
> Most documentation for hibernation explicitly cautions about the btrfs
> situation, but use of bcache is less common and as a result generally
> isn't covered.

Could you maybe add a warning to Documentation/bcache.txt? I think this
would have saved me.

Best,
-Nikolaus

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