linux-ext4 - Re: [patch] fs: fix deadlocks in writeback_if

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Date:	Mon, 29 Nov 2010 19:50:46 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...nel.dk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-ext4 <linux-ext4@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, Eric Sandeen <sandeen@...hat.com>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [patch] fs: fix deadlocks in writeback_if_idle

Excerpts from Andrew Morton's message of 2010-11-24 17:47:40 -0500:
> On Tue, 23 Nov 2010 07:34:07 -0500
> Chris Mason <chris.mason@...cle.com> wrote:
> 
> > For btrfs there's only one bdi per SB, but for most everyone else a disk
> > with a bunch of partitions is going to have multiple filesystems on the
> > same bdi.
> 
> um, please explain why that wasn't idiotic?  The BDI is a
> representation of a backing device and it's *supposed* to provide
> visibility into what's happening against other partitions on the same
> device.  Creating a BDI per SB (it didn't even occur to me to think
> that a filesystem was even able to do this) breaks that.
> 

We don't really have visibility into the other partitions, we all just
pretend they aren't there (this patch being the most recent example).
Yes, it does help prevent N flushers seeking around on the drive but it
does cause problems too.

How is the btrfs one-bdi-per-super different from device mapper's one
bdi per logical volume?  We're all idiots together I suppose.

As for having multiple bdis per FS, that was always a long term goal of
mine when Jens was setting up the new flushers.  I didn't want to
confuse the initial implementation with it though.

-chris
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