Hello Neil,
> Leal,
>
> ZFS uses the DNLC. It still provides the fastest
> lookup of <directory, name> to vnode.
Ok, so the whole concept remains true? We can tune the DNLC and expect the
same behaviour on ZFS?
> The DNLC is kind of LRU. An async process will use a
> rotor to move
> through the hash chains and select the LRU entry but
> will select first
> negative cache entries and vnodes only referenced by
> the DNLC.
> Underlying this ZFS uses the ZAP and Fat ZAP to store
> the mappings.
Here i did not understand very well. You are saying that ZFS uses DNLC just
for one level?
>
> ZFS does not use the 2nd level DNLC which allows
> caching of directories.
> This is only used by UFS to avoid a linear search of
> large directories.
What is the ZFS way here? One of the points of my question is exactly that...
in an environment with many directories with *many* files, i think ZFS would
has the *same* problems too.
So, having directories cache on DNLC could be a good solution. Can you explain
how ZFS handles the performance in directories with hundreds of files?
There is a lot of docs around UFS/DNLC, but for now i think the only doc about
ZFS/ARC and DNLC is the source code. ;-)
>
> Neil.
Thanks a lot!
I was thinking in tune DNLC to have as many metadata (directories and files)
as i can, to minimize lookups/stats and etc (in NFS there is a lot of getattr
ops). So we could have *all* the metadata cached, and use what remains in
memory to cache data.
Maybe that kind of tuning would be usefull for just a few workloads, but could
be a *huge* enhancement for that workloads.
Leal
-- posix rules --
[http://www.posix.brte.com.br/blog]
>
> On 10/30/08 04:50, Marcelo Leal wrote:
> > Hello,
> > In ZFS the DNLC concept is gone, or is in ARC too?
> I mean, all the cache in ZFS is ARC right?
> > I was thinking if we can tune the DNLC in ZFS like
> in UFS.. if we have too *many* files and
> > directories, i guess we can have a better
> performance having all the metadata cached, and that
> > is even more important in NFS operations.
> > DNLC is LRU right? And ARC should be totally
> dynamic, but as in another thread here,
> > i think reading a *big* file can mess with the
> whole thing. Can we hold an area in memory
> > for DNLC cache, or that is not the ARC way?
> >
> > thanks,
> >
> > Leal.
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