[GSoC] HAMMER compression and new unionfs

Naohiro Aota naota at elisp.net
Wed Mar 30 09:42:11 PDT 2011

Alex Hornung <ahornung at gmail.com> writes:

> On 28/03/11 16:48, Naohiro Aota wrote:
>> about implementing a new unionfs:
>>> The current unionfs is completely broken as it relies on the whiteout
>>> VFS technique which is not supported by HAMMER.
>> - Then main work would be to implement whiteout feature on HAMMER?
>> - or rewrite unionfs code not to depend on whiteout technique?
> I've now changed the text on the projects page to reflect that we don't
> want a new unionfs using whiteout. So the aim of this project is to
> develop a unionfs from scratch avoiding techniques which require messing
> with other filesystems.

I've read current unionfs implement roughly, but I can't imagine how I
implement unionfs without whiteout. Are there any way to do so?

Or I should write code to support whiteout not on VFS side but on
unionfs side?

> About the VFS you can refer to the book "Design & Implementation of
> FreeBSD". Things have changed, but it gives a good general perspective
> of how things work. The namecache, most interesting part for unionfs,
> has no real documentation, only inline with the code.

I grep'ed unionfs source directory with "namecache", but got no result..
Could you tell me where it is used?


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