Hello Guys,
On one of my SSD based NAS, that has 128GB RAM, i see this:
What is ARC size and how do i know if it is working as expected or maybe need fine tuning? What does it exactly tells?
ARC is effectively a read cache, entirely in RAM.
When you read data from your pools the data sticks around in the ARC, frequently read stuff is kept there.
CORE and SCALE are both set up to let ARC grow to almost all of your free RAM, the sentiment is that free RAM is wasted RAM.
Your usage looks fine, given that you have 128 GB in the system.
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Great. I see people talking about changing the ARC hit ratio. Should i consider tuning anything on that?
That doesn’t make a whole lot of sense to me.
Hit ratio is when a read can be handled with the data cached in the ARC. The ratio is what it is and is partially a result of the amount of RAM you have and your usage pattern.
If you keep reading the same data over and over, and that data fits in your available amount of ARC, your hit ratio will be high.
If, however, you are more a read once and move on type of usage case, your hit ratio will naturally trend lower because the system hasn’t had a chance to cache the data before the first (and only) time it’s read.
So, case in point, if you use it for watching movies and you only watch a movie once and then move on to the next one, your ARC isn’t really going to help. If you have kids who watch the same episode 100 times in a row however, that episode is likely going to be cached in your ARC, meaning a higher hit ratio.
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Makes sense.