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

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Message-Id: <20081217115325.3312858a.akpm@linux-foundation.org>
Date:	Wed, 17 Dec 2008 11:53:25 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	adilger@....com, chris.mason@...cle.com, 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, 17 Dec 2008 08:23:44 -0500
Christoph Hellwig <hch@...radead.org> wrote:

> FYI: here's a little writeup I did this summer on support for
> filesystems spanning multiple block devices:
> 
> 
> -- 
> 
> === Notes on support for multiple devices for a single filesystem ===
> 
> == Intro ==
> 
> Btrfs (and an experimental XFS version) can support multiple underlying block
> devices for a single filesystem instances in a generalized and flexible way.
> 
> Unlike the support for external log devices in ext3, jfs, reiserfs, XFS, and
> the special real-time device in XFS all data and metadata may be spread over a
> potentially large number of block devices, and not just one (or two)
> 
> 
> == Requirements ==
> 
> We want a scheme to support these complex filesystem topologies in way
> that is
> 
>  a) easy to setup and non-fragile for the users
>  b) scalable to a large number of disks in the system
>  c) recoverable without requiring user space running first
>  d) generic enough to work for multiple filesystems or other consumers
> 
> Requirement a) means that a multiple-device filesystem should be mountable
> by a simple fstab entry (UUID/LABEL or some other cookie) which continues
> to work when the filesystem topology changes.

"device topology"?

> Requirement b) implies we must not do a scan over all available block devices
> in large systems, but use an event-based callout on detection of new block
> devices.
> 
> Requirement c) means there must be some version to add devices to a filesystem
> by kernel command lines, even if this is not the default way, and might require
> additional knowledge from the user / system administrator.
> 
> Requirement d) means that we should not implement this mechanism inside a
> single filesystem.
> 

One thing I've never seen comprehensively addressed is: why do this in
the filesystem at all?  Why not let MD take care of all this and
present a single block device to the fs layer?

Lots of filesystems are violating this, and I'm sure the reasons for
this are good, but this document seems like a suitable place in which to
briefly decribe those reasons.

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