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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Theodore Tso <tytso@mit.edu>
Subject: Re: [Tux3] Tux3 report: Tux3 Git tree available
Date: Mon, 16 Mar 2009 21:14:13 +1100
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Cc: Daniel Phillips <phillips@phunq.net>, linux-fsdevel@vger.kernel.org,
       tux3@tux3.org, Andrew Morton <akpm@linux-foundation.org>,
       linux-kernel@vger.kernel.org
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On Monday 16 March 2009 17:38:30 Theodore Tso wrote:
> Dave,
>
> It wasn't my intention to say that XFS was bad; in fact, I thought I
> was actually complementing XFS by talking about some of the advanced
> features that XFS had (many of which I have always said that ext3 has,
> and some of which ext4 still does not have, and probably never will
> have).  I stand corrected on some of the details that I got wrong.
> What I was trying to say was that *if* (and perhaps I'm
> misunderstanding fsblock) that fsblock is requiring that as soon as a
> page is dirty, fsblock requests the filesystem to assign a block
> allocation to the buffers attached to the dirty page, that this would
> spike out delayed allocation, which would be unfortunate for *both*
> ext4 and XFS.
>
> But maybe I'm misunderstanding what fsblock is doing, and there isn't
> a problem here.

Yeah, Dave's understanding of fsblock is correct. I might have stated
things confusingly... fsblock allocates the in-memory fsblock metadata
structure (~= struct buffer_head) at the time of block dirtying. It
also asks the filesystem to respond to the dirtying event appropriately.
In the case of say ext2, this means allocating a block on disk. Wheras
XFS does the delalloc/reserve thing (yes, XFS appears to be working
with fsblock well enough to get this far).

fsblock really isn't too much different to buffer_head from an abstract
capability / functionality point of view except that it is often more
strict with things where I feel it makes sense.

So for this particular example; in buffer.c, buffers do tend to be
allocated when a page is dirtied, but not always, and even when they are,
they can get reclaimed while the page is still dirty. fsblock tigtens
this up.

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