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