DataStax Academy FAQ

DataStax Academy migrated to a new learning management system (LMS) in July 2020. We are also moving to a new Cassandra Certification process so there are changes to exam bookings, voucher system and issuing of certificates.

Check out the Academy FAQ pages for answers to your questions:


question

chandrasekar.b03_190734 avatar image
chandrasekar.b03_190734 asked ·

What is off-heap memory?

What is off heap memory? How and why is it used for storing bloom filter, row cache, key cache and partition summary?

I was reading the read path on the documentation. During the read path on cassandra , read starts by moving through several stages like row cache, bloom filter. In most of these stages, the subsets of data are stored in memory "Off heap memory". I just need to know what is it? How is it storing the subsets and why not to store it in disk where the stables are being stored? What is the benefit of storing in off heap memory?


cassandrajvm
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bettina.swynnerton avatar image
bettina.swynnerton answered ·

Hi,

Off-heap store refers to (serialized) objects that are stored in memory, outside the heap, and not subject to garbage collections. As this is still in memory, access is faster than on disk.

A main reason to store off-heap is to offload the heap and keep its size smaller, which in turn helps to keep garbage collection pauses shorter.

I hope this helps!

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