[Lustre-devel] statahead feature

Peter Braam Peter.Braam at Sun.COM
Thu Jul 24 09:27:28 PDT 2008

I strongly agree with this.  A good way to verify if we have a favorable
implementation is to see if it can be ported to other OS's.


On 7/24/08 4:19 AM, "Alex Zhuravlev" <Alex.Zhuravlev at Sun.COM> wrote:

> Hi,
> due to some experiments with dcache related code we've been doing with shadow
> and others, it became clear that statahead code is quite complicated. probably
> for no reason. the most hard part to follow is interaction with dcache. the
> feature does number of complex things and make other parts (like
> ll_lookup_it())
> harder to follow too.
> after amount of discussions with people we'd like to share our vision on the
> feature and propose slightly different solution.
> we think statahead should do nothing with dcache. it's about inodes and
> attributes
> only. thus, it would be good to decouple it from dcache. the only thing
> statahead
> should do is:
> 1) detect statahead is needed (policy, out of the message's scope)
> 2) scan part of directory (probably using readdir(), skip RPCs)
> 3) finds/creates inodes for found fids
> 4) lock these inodes (notice we propose to use inodes as a serialization point
>     so that lockless getattr can be used)
> 5) issue getattr RPCs (probably lockless)
> 6) unlock inodes upon getattr's completion
> then stat(2) is called, it first has to lookup fid by name. for this we can
> use
> pagecache just filled with MDS_READDIR. if directory isn't being modified at
> the
> time, then entries will be there and we can create dentries in the dcache.
> they
> will be valid till UPDATE lock is cancelled - no even LOOKUP lock is needed.
> another possible thing for optimization is lockless getattr. given most of
> supported
> kernel don't pass intent to ->getattr(), it's possible that stat(2) needs two
> RPCs:
> one in ll_lookup_it() and another in ll_getattr_it() as lock is released
> between them.
> stat(2) gives no warranty about attributes, it gives a shot of them.
> attributes can
> change right before userspace application get them. so, why don't we introduce
> some
> simple mechanism making attributes valid for short time at least for process
> executed
> lookup. this could help statahead as well, we think.
> comments? suggestions?
> thanks, Alex
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