Wednesday Oct 12, 2011

ARC- Adaptive Replacement Cache

I know, I know, I told you I was going to talk about the very important VDEVs next, but this other article came up in another blog, and it’s a rather good read about the ZFSSA cache system, called our ARC, or Adaptive Responsive Cache.

So, if you want to learn more about the ARC in a ZFSSA, go check it out. Our ARC has two levels. Level 1 ARC is our RAM. Almost the entire RAM in a ZFSSA is used for data caching, and that’s the ARC, or L1ARC. Now, we go further by having a L2ARC. Once RAM is full, our L2ARC can hold even more cache by using any Readzillas you have in the system. That’s right; our Readzillas SSDs are the L2ARC. We use SSDs for cache, not as storage space. (Logzillas, on the other hand, are for fast synchronous write acknowledgements, and have nothing to do with ARC at all).

So a 7420 with 512GB memory and four Readzillas has about a 500GB L1ARC and a 2TB L2ARC to use as an Adaptive Responsive Cache to work with. 500GB of that 2.5TB of space will be nano-second speed while 2TB of it will be micro-second speed. Still much faster than the milli-second speed you get when you have to get data off a hard drive.

So Cache is cool, and it’s nice to have a high cache hit ratio, and it’s easier to have a high cache hit ratio if you have more cache, right? With the new, lower priced Readzillas, this should be easier to do.

Now, this other blog I’m pointing you to says we call our cache something else, but don’t worry about it, we use the name “Adaptive responsive Cache” in the Oracle ZFSSA world.

Go check out:
http://www.c0t0d0s0.org/archives/5329-Some-insight-into-the-read-cache-of-ZFS-or-The-ARC.html

Ok, VDEVs will come next!

Maybe.

Steve

About

This blog is a way for Steve to send out his tips, ideas, links, and general sarcasm. Almost all related to the Oracle 7000, code named ZFSSA, or Amber Road, or Open Storage, or Unified Storage. You are welcome to contact Steve.Tunstall@Oracle.com with any comments or questions

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