Re: Notes on support for multiple devices for a single filesystem

["Liu Hui" <onlyflyer@gmail.com>]

A very interesting article wrotete by Jeff Bonwick for Andrew --
"Rampant Layering Violation?"

http://blogs.sun.com/bonwick/entry/rampant_layering_violation

2008/12/18 Andrew Morton <akpm@linux-foundation.org>:
> On Wed, 17 Dec 2008 08:23:44 -0500
> Christoph Hellwig <hch@infradead.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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-- 
Thanks & Best Regards
Liu Hui
--