Re: dm-cache: can the same cache be used with multiple origin devices?

[Mike Snitzer <snitzer@redhat.com>]

On Mon, Jul 15 2013 at  3:01pm -0400,
Mears, Morgan <Morgan.Mears@netapp.com> wrote:

> Hi,
> 
> In reference to dm-cache: can the same cache and metadata devices be
> used with multiple origin devices?  This can be configured, and we've
> done some tests that appear to show that it works - we're looking for
> confirmation (or otherwise).

It is _not_ supported.
 
> Here's an example test setup to clarify -- ssd_metadata and ssd_blocks
> are being used to cache sdc and sdd.  In testing, different patterns
> were written to areas of sdc_cached and  sdd_cached; afterwards, the
> contents of sdc and sdd were as expected.
> 
> dmsetup create sdc_cached --table '0 4194304 cache /dev/mapper/ssd_metadata /dev/mapper/ssd_blocks /dev/sdc 512 1 writethrough default 0'
> dmsetup create sdd_cached --table '0 4194304 cache /dev/mapper/ssd_metadata /dev/mapper/ssd_blocks /dev/sdd 512 1 writethrough default 0'

Interesting.

The current cache target obviously fails to detect that the metadata or
data devices are already in use.  But that doesn't mean it is safe to
utilize the cache in this mode (I'll have a think about where the code
will break down).  But the cache is managed/written in a manner that
only assumes a single backing origin for each cache.

> Our thinking is that using one large cache for multiple origin devices
> will result in a more efficient use of flash resources than statically
> partitioning the flash amongst the origins.  Also, there's no need to
> repartition or buy another SSD to start caching a newly-added origin.

We originally thought about allowing a shared cache (metadata+data) for
N origin devices but decided against it to reduce the complexity of the
initial target code.