[Lustre-devel] WBC HLD outline

[Nikita Danilov]

2009/4/8 Alexander Zarochentsev <Alexander.Zarochentsev at sun.com>

> Hello Nikita!
>
> On 7 April 2009 11:50:29 Nikita Danilov wrote:
> > 2009/4/7 Alex Zhuravlev <bzzz at sun.com>
> >
> > > >>>>> Andreas Dilger (AD) writes:
> >
> > Hello,
> >
>

[...]


> > $ echo > a/b/c/d/file # truncate secret data
> > $ chmod 777 a # relax permissions
> >
> > Note that here an ordering between data and meta-data updates on
> > _different_ objects is important.
>
> If we only guarantee no reordering in MD updates, Lustre behavior would
> be like ext3 without data journalling? I think it is not terrible.


It's not terrible, but it is non-intuitive, in my opinion. More enlightened
file systems, like ZFS, reiser4, and NTFS provide stronger consistency
guarantees, ignoring the petty distinctions between data and meta-data. :-)

But even limiting consistency to meta-data leaves some issues opened. For
example, think about an md proxy server acting as a WBC client for a higher
tier server. To be efficient such proxy might need to cache very large
amount of meta-data, and it most likely cannot afford to keep a log of all
operations. In this situation, when a lock on a top-level directory gets a
blocking AST, proxy would have --to guarantee ordering of visible meta-data
updates-- to write back all cached dirty meta-data under this directory
before the lock can be cancelled, which might result in unacceptable
latency.

>
> > > thanks, Alex
> >
> > Nikita.
>
> --
> Alexander "Zam" Zarochentsev
>

Nikita.
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