[OpenIndiana-discuss] ZFS with Dedupication for NFS server
Edward Ned Harvey
openindiana at nedharvey.com
Mon Apr 25 03:25:08 UTC 2011
> From: Roy Sigurd Karlsbakk [mailto:roy at karlsbakk.net]
> > With the 8T storage... Add 8-16G ram to your server on top of your
> > baseline.
> I never got around to filling it. Already at about 1.2TB fill, it was dead slow,
> and for that the 8GB RAM should suffice.
> > I'm not sure how to calculate extra ram requirements for l2arc.
> AFAIK, L2ARC will just "add more, slower RAM"...
Sorry, both of these statements are inaccurate.
(1) Well, I'll drop the subject of 8G being enough ram. If I say any more about that, I'll just be repeating myself. So I'll only repeat one part: 8G is not enough if you have dedup and l2arc enabled and expect it to perform reasonably. I don't even run a laptop with less than 8G of ram anymore.
(2) L2arc is not simply a slower extension of L1arc as you seem to be thinking. Every entry in the L2arc requires an entry in the L1arc. I don't know what the multiplier ratio is, but I hear something between 10x and 20x. So if you have for example 20G of L2arc, that would consume something like 1-2G of ram.
Oddly enough, just google for this: L2ARC memory requirements
And what you see is --- A conversation in which you, Roy, and I both participated. And it was perfectly clear that you require RAM consumption in order to support your L2ARC. So, I really don't know how any confusion came about here... You should be solidly aware by now, that enabling L2ARC comes with a RAM cost.
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