linux-kernel - Re: [PATCH 25/27] nilfs2: block cache for garbage collection

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Message-Id: <20080918.040945.32654226.ryusuke@osrg.net>
Date:	Thu, 18 Sep 2008 04:09:45 +0900 (JST)
From:	Ryusuke Konishi <konishi.ryusuke@....ntt.co.jp>
To:	joern@...fs.org
Cc:	akpm@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, kihara.seiji@....ntt.co.jp,
	amagai.yoshiji@....ntt.co.jp
Subject: Re: [PATCH 25/27] nilfs2: block cache for garbage collection

On Wed, 17 Sep 2008 16:41:47 +0200, Jörn Engel wrote:
> On Mon, 15 September 2008 04:08:22 +0900, Ryusuke Konishi wrote:
> > 
> > This adds the cache of on-disk blocks to be moved in garbage
> > collection.  The disk blocks are held with dummy inodes (called
> > gcinodes), and this file provides lookup function of the dummy inodes,
> > and their buffer read function.
> 
> Nice explanation.  Can you add it to the comment header at the top of
> the file?  Unlike the GPL preample, it actually helps non-lawyers. ;)

Well, I see. ;)

> Using dummy inodes is... unusual.  Why can you not use the actual inodes
> those blocks belong to?

Because we have to treat blocks that belong to a same file but have
different checkpoint numbers.  (NILFS2 keeps up multiple
checkpoints/snapshots across GC)

Of course, if the standard inode hash is applicable, I prefer it.
ilookup5 or its variant may be applicable for this.

If so, the remaining problem would be the lock dependencies as you
mentioned before.

> Or alternatively a single inode that simply
> covers the complete physical device?
> 
> Jörn

NILFS2 writes GC blocks per file like other files, so the per file
caches (even separate inodes) are convienient for this end.

Ryusuke
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