linux-kernel - Re: Notes on support for multiple devices for a single filesystem

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Date:	Wed, 17 Dec 2008 16:41:58 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Andreas Dilger <adilger@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>, sfr@...b.auug.org.au,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: Notes on support for multiple devices for a single filesystem

On Wed, 2008-12-17 at 14:24 -0700, Andreas Dilger wrote:

> I can't speak for btrfs, but I don't think multiple device access from
> the filesystem is a "layering violation" as some people comment.  It
> is
> just a different type of layering.  With ZFS there is a distinct layer
> that is handling the allocation, redundancy, and transactions (SPA,
> DMU)
> that is exporting an object interface, and the filesystem (ZPL, or
> future
> versions of Lustre) is built on top of that object interface.

Clean interfaces aren't really my best talent, but btrfs also layers
this out.  logical->physical mappings happen in a centralized function,
and all of the on disk structures use logical block numbers.

The only exception to that rule is the superblock offsets on the device.

-chris


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