dm-cache selective promotion of block

dm-cache selective promotion of block

[Mahmoud Hanafi]

[-- Attachment #1.1: Type: text/plain, Size: 294 bytes --]

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?

-- 
Mahmoud Hanafi
NASA Ames Research Center
Mail Stop 258-6
Bldg. 258
P.O. Box 1
Moffett Field, CA 94035



[-- Attachment #1.2: S/MIME Cryptographic Signature --]
[-- Type: application/pkcs7-signature, Size: 7174 bytes --]

[-- Attachment #2: Type: text/plain, Size: 0 bytes --]

Re: dm-cache selective promotion of block

[Mike Snitzer]

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

Re: dm-cache selective promotion of block

[Joe Thornber]

On Thu, Dec 17, 2015 at 11:11:18AM -0800, Mahmoud Hanafi 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?

Not at the moment.

Could expand more on why you feel the need for this?  Is this
something you want just for experimentation or would you want to use
it in production?  How do you know which blocks you want to pin?  If
the pinned blocks are in large regions do you actually just want to
build a device that is composed of linear mappings to a mixture of
spindle and ssd devices?

- Joe