linux-kernel - Re: [Bcache v13 11/16] bcache: Core btree code

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Message-ID: <20120523172435.GB18143@google.com>
Date:	Wed, 23 May 2012 10:24:35 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, agk@...hat.com
Subject: Re: [Bcache v13 11/16] bcache: Core btree code

Hello,

On Wed, May 23, 2012 at 01:34:03AM -0400, Kent Overstreet wrote:
> On Tue, May 22, 2012 at 10:20:54PM -0700, Tejun Heo wrote:
> > Hmmm... I would prefer it to be defined explicitly as union.  It's
> > rather easy to define it incorrectly (ie. using struct bkey) and then
> > pass it around expecting it to have the pad.
> 
> Thing is, things don't expect the pad - bkeys are normally just in a big
> chunk of memory concatenated together, and the same functions have to
> work both with those and with bare bkeys the code occasionally
> manipulates.

Hmmm... so it's actually just padding?  Padding for what?  I thought
it was there to provide space for ptr[], no?

> > I'm a bit confused.  Cache device or cached device?  Isn't the key
> > dev:offset:size of the cached device?
> 
> No - bkey->key is the offset on the cached device, PTR_OFFSET is on the
> cache.
> 
> Confusing, I know. Any ideas for better terminology?

Double confused by the "no" and the following sentence seemingly
agreeing with what I wrote.  So, bkey->key indexes the backend device
- the slow big disk and the associated PTRs point into the fast SSD
caching device, right?  If so, I think 'key' is fine, that's the only
thing which can be key anyway.  As for PTR_XXX, maybe something which
can signify they're the cache would be nicer but with proper comments
I don't think it's big deal.

Thanks.

-- 
tejun
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