linux-kernel - Re: introduce dm-snap-mv

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Message-Id: <201010080622.58095.phillips@phunq.net>
Date:	Fri, 8 Oct 2010 06:22:57 -0700
From:	Daniel Phillips <phillips@...nq.net>
To:	Christoph Hellwig <hch@....de>
Cc:	McPacino <mcpacino@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Nick Piggin <npiggin@...nel.dk>
Subject: Re: introduce dm-snap-mv

On Friday 08 October 2010, Christoph Hellwig wrote:
> On Fri, Oct 08, 2010 at 05:14:27PM +0800, McPacino wrote:
> > Hi Christoph,
> > 
> > I have to take care the cache problem If using the bio directly.
> > BHs can be released by kernel when necessary.
> > 
> > Is there any existing code using bio to read/write metadata
> > blocks? How do they handle the timing freeing bios? I really
> > wish to learn something form it.
> 
> If you actually need caching just use the pagecache, e.g.
> read_mapping_page to read in your data.  That completely abstracts
> away the underlying block size.

And that will automatically give him the PAGE_CACHE_SIZE objects he
wants.  I still don't understand why his model cannot be generalized
to arbitrary block size specifiable at create time.

Regards,

Daniel
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