Storage Memory Usage Benchmark

Two components of etcd storage consume physical memory. The etcd process allocates an in-memory index to speed key lookup. The process’s page cache, managed by the operating system, stores recently-accessed data from disk for quick re-use.

The in-memory index holds all the keys in a data structure, along with pointers to the on-disk data (the values). Each key in the B-tree may contain multiple pointers, pointing to different versions of its values. The theoretical memory consumption of the in-memory index can hence be approximated with the formula:

where c1 is the key metadata overhead and c2 is the version metadata overhead.

Page cache memory is managed by the operating system and is not covered in detail in this document.

etcd version

GCE n1-standard-2 machine type

• 7.5 GB memory

In this test, we only benchmark the memory usage of the in-memory index. The goal is to find c1 and mentioned above and to understand the hard limit of memory consumption of the storage.

Based on the result, we can calculate c1=120bytes, c2=30bytes. We only need two sets of data to calculate c1 and , since they are the only unknown variable in the formula. The c1=120bytes and c2=30bytes are the average value of the 4 sets of c1 and we calculated. The key metadata overhead is still relatively nontrivial (50%) for small key-value pairs. However, this is a significant improvement over the old store, which had at least 1000% overhead.

The overall memory usage captures how much RSS etcd consumes with the storage. The value size should have very little impact on the overall memory usage of etcd, since we keep values on disk and only retain hot values in memory, managed by the OS page cache.

Based on the result, we know the value size does not significantly impact the memory consumption. There is some minor increase due to more data held in the OS page cache.