ZFS obn FreeBSD hardware model for 48 or 96 sata3 paths...

[Bob Friesenhahn]

On Mon, 19 Sep 2011, Jason Usher wrote:
>
> How does one make sure that all metadata and directories are cached 
> in RAM?  Just run a 'find' on the filesystem, or a 'du' during the 
> least busy time of day ?  Or is there a more elegant, or more direct 
> way to read all of that in ?

Caching occurs due to normal use and it is best to rely on that until 
proven otherwise.

> Further, if this (small files, lots of them) dataset benefits a lot 
> from having the metadata and dirs read in, how can I KEEP that data 
> in the cache, but not cache the file data (as you suggest, above) ?

Modern zfs includes tunables to decide how metadata and file data 
caching should be handled.  The main reason to disable file data 
caching would be for cases where the data is only accessed once such 
as when data is normally written out once to whole files or read just 
once with a well-behaved algorithm.  Video streaming servers may 
disable file caching if the number of streams served would cause the 
cache size to grow to huge (yet insufficient) proportions.

> Can I explicitly cache metadata/dirs in RAM, and cache file data in L2ARC ?

Again, it is best to rely on the caching algorithm until an actual 
problem has been found.  The ZFS ARC will optimize its caching based 
on use.  Less often used data will end up being migrated from the 
RAM-based ARC to L2ARC.

Bob
-- 
Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/