Re: what's the exact meaning of cap?

[wchen <wchen@tnsoft.com.cn>]

hi, Farnum,

Thank you very much! :-)


On Thu, 2010-12-02 at 00:11 +0800, Gregory Farnum wrote:
> On Tue, Nov 30, 2010 at 7:50 PM, wchen <wchen@tnsoft.com.cn> wrote:
> > hi, Sage,
> >
> > I'm reading the source code of ceph. Recently I'm very confused with
> > struct InodeCap's issued, implemented, wanted fields. I just have a
> > preliminary understanding of them, but what's the exact difference
> > between them? there are lots of code to reflect the relation between
> > them. Can you explain it for me? or any documents about it?
> Unfortunately we don't have a lot of documentation about the source
> code itself. Sage's thesis (which is available on the website) is the
> best resource, and does describe caps some.
> 
> Briefly, caps are short for capabilities. They are issued by the MDS
> to clients to describe what the client is allowed to do with an inode
> and its associated file. There are capabilities to, for instance, let
> the client buffer writes, cache reads, and adjust certain kinds of
> metadata (mtime, et al).
> The wanted capabilities are caps the client wants but doesn't have.
> Issued caps are ones the client has been granted, and (IIRC)
> implemented caps are a subset of the issued caps describing which caps
> the client has actually made use of. It's useful for determining
> whether the client has dirty metadata and whatnot. :)
> -Greg

-- 
best regards,
Kevin Chen (陈巍)

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