[Lustre-devel] readdir for striped dir

[Nikita Danilov]

On 23 March 2010 14:29, Tom.Wang <Tom.Wang at sun.com> wrote:
> Hello,

Hello,

[...]

>
> LMV will use new hash function to select stripe object (mdc), which could be
> independent with the one
> used in the storage.  In mdc level, it just need to map the entries of each
> dir stripe object in the cache,
> we can index the cache in anyway as we want, probably hash order (as the
> server storage) is a good choice,
> because client can easily find and cancel the pages by the hash in later
> dir-extent lock. Note: Even in this case,
> client does not need to know server hash scheme at all, since server will
> set the hash-offset of these pages, client just
> need to put these pages on the cache by hash-offset.
> Currently, the cache will only be touched by readdir.  If the cache will be
> used by readdir-plus later, i.e. we need locate
> the entry by name, then client must use the same hash as the server storage,
> but server will tell client which hash function
> it use.  Yes, different hash per dirstripe should not be a problem here.

If I understand correctly, the scheme has an advantage of cleaner
solution for readdir scalability than "hash adjust" hack of CMD3 (see
comment in lmv/lmv_obd.c:lmv_readpage()). The problem, to remind, is
that if a number of clients readdir the same split directory, they all
hammer the same servers one after another, negating the advantages of
meta-data clustering. The solution is to cyclically shift hash space
by an offset depending on the client, so that clients load servers
uniformly. With 2-level hashing, this shifting can be done entirely
within new LMV hash function.

>
> Thanks
> WangDi

Thank you,
Nikita.