[zfs-discuss] Arc limiting guide

Gordan Bobic gordan.bobic at gmail.com
Tue May 7 15:00:34 EDT 2013

On 05/07/2013 01:26 AM, Schlacta, Christ wrote:
> I'm looking for a way to determine how much arc is useful. Obviously
> with primary cache= all, ram up to the size of your data set would be
> useful. Instead, I refer to the case of primary cache= metadata. I'd
> imagine the same guidelines can be applied to l2arc with similar
> guidelines.
> This is a common issue. How much cache do I need/will be useful? I'd
> like to have an entry added to the faq once a consensus can be reached.
> Similarly, I'd like a faq entry for zil sizes.

I think you are making the issue sound more complicated than it actually 
is. The answer to your question ultimately depends on the workload. The 
general rule of thumb is that for near-optimal cache efficiency you need 
to keep your last 5 minutes worth of data in the caches. Since disks 
have grown much faster than RAM sizes, you need to extend your ARC with 
an L2ARC to get back to the 5 minutes of data figure. The important 
point to take from this, however, is that how much cache you need is 
completely dependent on the workload.


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