linux-ext4 - Re: [PATCH 1/4] vfs: vfs-level fiemap interface

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Message-ID: <20080920164759.GY27404@josefsipek.net>
Date:	Sat, 20 Sep 2008 12:47:59 -0400
From:	Josef 'Jeff' Sipek <jeffpc@...efsipek.net>
To:	Theodore Tso <tytso@....edu>
Cc:	Mark Fasheh <mfasheh@...e.com>,
	Christoph Hellwig <hch@...radead.org>,
	Anton Altaparmakov <aia21@....ac.uk>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	akpm@...uxfoundation.org
Subject: Re: [PATCH 1/4] vfs: vfs-level fiemap interface

On Tue, Sep 16, 2008 at 05:31:07PM -0400, Theodore Tso wrote:
> On Mon, Sep 15, 2008 at 10:53:05AM -0700, Mark Fasheh wrote:
> > On Mon, Sep 15, 2008 at 10:49:48AM -0400, Christoph Hellwig wrote:
> > > I agree to you (or someone elses - don't remember anymore) suggestion
> > > to put in more padding so we can add fields later.  I strongly disagree
> > > putting in features now that we neither have a user, nor a usecase or
> > > testcase for.
> > 
> > So, how about another 64 bits of padding in struct fiemap_extent? That could
> > help cover future uses like compression, which might require another 64 bit
> > offset field - we only have 32 bits of reserved space there right now.
> 
> What I'd recommend is a 56 byte structure:

Why not just make it 64 bytes? Sure, that's 8 extra bytes, but I find the
power-of-2 size (and the extra space) comforting. (AFAIK, slab allocators
will give you 64 bytes anyway; and I expect something similar on the
user-space side of things.)

...
> Yeah, it's a little extra memory per extent, but filesystems seem to
> always invent new things, and it seem spretty clear that we have at
> least two extensions on deck (compression, multiple storage devices)
> both of which have at least one implementation that are either in the
> kernel or will likely enter the kernel.  So it's likely that there is
> something that we may have missed, and leaving a little extra space
> doesn't actually cost us that much.

Right.

Josef 'Jeff' Sipek.

-- 
Once you have their hardware. Never give it back.
(The First Rule of Hardware Acquisition)
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