CODEWITHSUNDEEP
azure-data-factoryazure-data-explorer
Dhiraj
Dhiraj

Reputation: 3696

Meaning of ADX Cache utilization more than 100%

We see Cache utilization dashboard for an ADX cluster on Azure portal, but at times I have noticed that this utilization shows up to be more than 100%. I am trying to understand how to interpret it. Say , for example , if cache utilization shows up as 250% , does it mean that 100% of memory cache is utilized and then beyond that 150% disk cache is being utilized?

Upvotes: 1

Views: 1394

Answers (2)

Ziv Caspi
Ziv Caspi

Reputation: 76

Utilization > 100% means that there's not enough room in the (SSD) cache to hold all the data that the policy indicates should be cached. If auto-scale is enabled then the cluster will be scaled-out as a result.

The cache applies an LRU eviction policy, so that even when utilization exceeds 100% query performance will be as good as possible (though, of course, if queries constantly reference data more than what the cache can hold some performance degradation will be observed.)

Upvotes: 0

Yoni L.
Yoni L.

Reputation: 25965

as explained in the documentation for the Cache Utilization metric:

[this is the] Percentage of allocated cache resources currently in use by the cluster.

Cache is the size of SSD allocated for user activity according to the defined cache policy.

An average cache utilization of 80% or less is a sustainable state for a cluster.

If the average cache utilization is above 80%, the cluster should be scaled up to a storage optimized pricing tier or scaled out to more instances. Alternatively, adapt the cache policy (fewer days in cache).

If cache utilization is over 100%, the size of data to be cached, according to the caching policy, is larger that the total size of cache on the cluster.

Upvotes: 3

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