How well does bcache cope with frequently changing cache device?

How well does bcache cope with frequently changing cache device?

[Clemens Eisserer]

Hi,

I am thinking about working with a root-fs on an external disk which
can be shared amongst different computers.
For this use-case the internal SSD could be used as caching device,
speeding up the access to the relativly slow external storage.

What I wonder is how well bcache can cope with changing caching
devices? Great would be if bcache could detect when a backing decive
was used with a different caching device in the meantime and
automatically invalidate the old/stale caching device - but to keep
the cache unmodified when this was not the case.

Is this use-case possible with bcache?

Thank you in advance, Clemens

Re: How well does bcache cope with frequently changing cache device?

[Kent Overstreet]

On Sun, Sep 04, 2016 at 07:08:04PM +0200, Clemens Eisserer wrote:
> Hi,
> 
> I am thinking about working with a root-fs on an external disk which
> can be shared amongst different computers.
> For this use-case the internal SSD could be used as caching device,
> speeding up the access to the relativly slow external storage.
> 
> What I wonder is how well bcache can cope with changing caching
> devices? Great would be if bcache could detect when a backing decive
> was used with a different caching device in the meantime and
> automatically invalidate the old/stale caching device - but to keep
> the cache unmodified when this was not the case.
> 
> Is this use-case possible with bcache?

On its own bcache won't automatically invalidate the cached data - once a
backing device is associated with a certain cache, that backing device won't
come online until either the cache device is registered, or you detach it.

So you'll have to write your own scripts to check - "hey, is this backing device
associated with this cache device? if not, detach backing device, reformat cache
device, then attach cache device and backing device".