On 01/31/11 06:40 PM, Roy Sigurd Karlsbakk wrote:
----- Original Message -----
Even *with* an L2ARC, your memory requirements are *substantial*,
because the L2ARC itself needs RAM. 8 GB is simply inadequate for
your
test.
With 50TB storage, and 1TB if L2ARC, with no dedup, what amount of ARC
would you would you recommeend?
First off... a *big* caveat. I am *not* a tuning expert. We have
people in our company who can help you out from operational experience
if you want to configure a system like this -- one of them -- Richard
Elling -- is a frequently seen face around here. That said, I'm going
to respond from my *very* rough understanding of how these structures
play together....
So, I'd say: Alot. 1TB for ARC sounds like a rather largeish amount.
I don't know offhand the typical ratio of ARC -> L2ARC, but note that
every entry in the L2ARC requires at least some book-keeping in the ARC
(which is in RAM). I've seen people say that you can have anywhere from
10x RAM to 20x RAM for L2ARC. It sounds like this means between 50GB
and 100GB *just* for an L2ARC of this size.
That's without dedup.
And then, _with_ dedup, what would you recommemend?
make that 100TB of storage
With 100TB of storage, fully consumed, your DDT is going to need to be
about 500GB (assuming 64K block size, which may or may not be a good
average).
That whole DDT will fit into the L2ARC above, so you probably can get by
with just the 50-100GB of RAM. But I recommend allocating *more* than
that, because you really don't want *every* write to the DDT to go to
L2ARC, and you really *do* want to have some memory available for things
besides just the ARC.
Generally, this feels like a 256GB memory configuration to me.
Fundamentally, the best way to reduce the memory impact is to use dedup
much more sparingly, and configure a much smaller L2ARC.
You also need to analyze your workload to see if you'll benefit from
having L2ARC apart from the DDT itself.
- Garrett
_______________________________________________
zfs-discuss mailing list
[email protected]
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
