Re: dm-cache selective promotion of block

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Dec 17 2015 at  2:11pm -0500,
Mahmoud Hanafi <mahmoud.hanafi@nasa.gov> wrote:

> We are experimenting with dm-cache and would like to be able to
> promote a range of block and pin them in the cache device.
> 
> Are there any tool that can be used for this?

There isn't an interface to do that right now.  But it _should_ be
possible to add a DM message to easily promote an origin block range.
Pinning isn't something we really do at this point so that'd require
more work.

But it could look something like this:
dmsetup message <cache_dev> 0 promote_oblocks <start> <end>
dmsetup message <cache_dev> 0 pin_oblocks <start> <end>

and:
dmsetup message <cache_dev> 0 demote_oblocks <start> <end>
dmsetup message <cache_dev> 0 unpin_oblocks <start> <end>

(where <start> and <end> are in either cache_block_size units or 512b
sectors... not sure which would be best, sectors is a bit more
approachable)

I could see this getting a bit cumbersome, not to mention the smq cache
policy really _should_ promote if it makes sense to cache a block, but
we'll certainly need Joe's thoughts on this.

Mike