CODEWITHSUNDEEP
couchbase
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Reputation: 21

How to correctly calculate RAM requirement for a bucket in Couchbase

We have a bucket of about 34 million items in a Couchbase cluster setup of 6 AWS nodes. The bucket has been allocated 32.1GB of RAM (5482MB per node) and is currently using 29.1GB. If I use the formula provided in the Couchbase documentation (http://docs.couchbase.com/admin/admin/Concepts/bp-sizingGuidelines.html) it should use approx. 8.94GB of RAM.

Am I calculating it incorrectly? Below is link to google spreadsheet with all the details. https://docs.google.com/spreadsheets/d/1b9XQn030TBCurUjv3bkhiHJ_aahepaBmFg_lJQj-EzQ/edit?usp=sharing

Upvotes: 2

Views: 1012

Answers (1)

David Ostrovsky
David Ostrovsky

Reputation: 2481

Assuming that you indeed have a working set of 0.5%, which as Kirk pointed out in his comment, is odd but not impossible, then you are calculating the result of the memory sizing formula correctly. However, it's important to understand that the formula is not a hard and fast rule that fits all situations. Rather, it's a general guideline and serves as a good starting point for you to go and begin your performance tests. Also, keep in mind that the RAM sizing isn't the only consideration for deciding on cluster size, because you also have to consider data safety, total disk write throughput, network bandwidth, CPU, how much a single node failure affects the rest of the cluster, and more.

Using the result of the RAM sizing formula as a starting point, you should now actually test whether your working assumptions were correct. Which means putting real (or close to representative) load on the bucket and seeing whether the % of cache misses is low enough and the operation lacency is within your acceptable limits. There is no general rule for this, what's acceptable to some applications might be too slow for others.

Just as an example, if you see that under load your cache miss ratio is 5% and while the average read latency is 3ms, the top 1% latency is 100ms - then you have to consider whether having one out of every 100 reads take that much longer is acceptable in your application. If it is - great, if not - you need to start increasing the RAM size until it matches your actual working set. Similarly, you should keep an eye on the disk throughput, CPU usage, etc.

Upvotes: 1

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