linux-kernel - Re: [dm-devel] [RFC][PATCH] dm-cache: block level disk cache target for device mapper

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Message-Id: <200611302107.40418.jens.wilke@de.ibm.com>
Date:	Thu, 30 Nov 2006 21:07:39 +0100
From:	Jens Wilke <jens.wilke@...ibm.com>
To:	dm-devel@...hat.com
Cc:	"Eric Van Hensbergen" <ericvh@...il.com>,
	Eric Van Hensbergen <ericvh@...a.kernel.org>,
	ming@...s.ufl.edu, linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [RFC][PATCH] dm-cache: block level disk cache target for device mapper

On Thursday 30 November 2006 17:24, Eric Van Hensbergen wrote:
> On 11/30/06, Jens Wilke <jens.wilke@...ibm.com> wrote:
> > On Monday 27 November 2006 19:26, Eric Van Hensbergen wrote:
> >
> > If this is intended to speed up remote disks, is it possible that the cache content
> > can be paged out on local disks in low-mem situations?
> >
> 
> The main intent was to use local disks as cache to offload centralized
> remote disks.  The logic was that most systems have local disks, if
> only for swap -- so why not use them as a cache to help offload
> centralized storage.  While the in-memory page cache works perfectly
> fine in certain situations -- we were dealing with workloads in which
> the in-memory page-cache wasn't sufficient to hold all the data.

I derived from the code that the cache is actually another block device
and not memory. Maybe you should give a little more description and
a sample 'dmsetup create' statement.

> There are also some additional possibilities we've thought through and
> have been playing with including allowing the local disk cache to be
> persistent across reboots (with varying validation schemes).

Yes, I like the idea. It would be also possible to use NV-Ram 
as write back cache, for example in notebooks, to avoid 
disk spin-ups or to improve transaction performance in
enterprise applications.

Maybe it would also make sense to prefill the cache contents
on startup with a bulk I/O request?

Comments on the code:

- The data of the write back cache should be flushed on suspend and not
  in the destructor.
- You don't keep track of I/O on the fly to the cache that is mapped
  directly in cache_hit(). How do you make sure that this I/O is completed 
  before you replace a cache block?
- The cache block index is hashed, this means the cache data blocks are not 
  clustered. I don't think you can solve this problem with a proper hash function. 
  Perhaps you should consider a (B-)Tree structure for that.

Best,

Jens
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