Memtable type and size allocation

[Vi...@ril.com]

Dear Community,

In Cassandra 3.11.2, there are 3 choices for the type of Memtable allocation and as per my understanding, if I want to keep Memtables on JVM heap I can use heap_buffers and if I want to store Memtables outside of JVM heap then I've got 2 options offheap_buffers and offheap_objects.

What exactly is the difference between the 2 choices given for off-heap allocation?

Also, the permitted memory space to be used for Memtables can be set at 2 places in the YAML file, i.e. memtable_heap_space_in_mb and memtable_offheap_space_in_mb.

Do I need to configure some space in both heap and offheap, irrespective of the Memtable allocation type or do I need to set only one of them based on my Memtable allocation type i.e. memtable_heap_space_in_mb when using heap buffers and memtable_offheap_space_in_mb only when using either of the other 2 offheap options?

https://stackoverflow.com/questions/49874917/memtable-type-and-size-allocation

Thanks and regards,
Vishal Sharma
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Re: Memtable type and size allocation

[kurt greaves <ku...@instaclustr.com>]

Hi Vishal,

In Cassandra 3.11.2, there are 3 choices for the type of Memtable
> allocation and as per my understanding, if I want to keep Memtables on JVM
> heap I can use heap_buffers and if I want to store Memtables outside of JVM
> heap then I've got 2 options offheap_buffers and offheap_objects.

Heap buffers == everything is allocated on heap, e.g the entire row and its
contents.
Offheap_buffers is partially on heap partially offheap. It moves the Cell
name + value to offheap buffers. Not sure how much this has changed in 3.x
Offheap_objects moves entire cells offheap and we only keep a reference to
them on heap.

Also, the permitted memory space to be used for Memtables can be set at 2
> places in the YAML file, i.e. memtable_heap_space_in_mb and
> memtable_offheap_space_in_mb.

 Do I need to configure some space in both heap and offheap, irrespective
> of the Memtable allocation type or do I need to set only one of them based
> on my Memtable allocation type i.e. memtable_heap_space_in_mb when using
> heap buffers and memtable_offheap_space_in_mb only when using either of the
> other 2 offheap options?


Both are still relevant and used if using offheap. If not using an offheap
option only memtable_heap_space_in_mb is relevant. For the most part, the
defaults (1/4 of heap size) should be sufficient.