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High-Load Storage of Users’ Actions with ScyllaDB and HDDs : The story, Russia's biggest email provider, will present how they implemented ScyllaDB using Hard Disk Drives (HDDs), their hardware setup and low latencies they achieved.

As an email provider, Mail.Ru gives its millions of users the ability to view and manage an ‘action history – a time series of actions that are stored per email. Every action has a user field, such as the IP address from which the action was committed, the ID of the client who created the action, and finally the action ID, which represents whether a user replied to an email, uploaded an image, and so on. The API supports 65,000 peak writes per second and a peak of 50 reads per second.

Having started out with a homegrown solution, Mail.Ru started looking for alternative storage options based on a set of limitations:

  • Poor scalability, requiring too many nodes to handle increases in traffic
  • Built from scratch internally, poor maintainability
  • Lack of essential features, such as secondary indexes, tunable replication, and query language

“We saved at least $150,000 of capital expenses per petabyte.”
– Kirill Alexseev, Software Engineering Technical Lead, Mail.Ru

The team decided to go with ScyllaDB as storage for user actions. Today, Mail.Ru runs ScyllaDB on bare metal in two datacenters, with 4 nodes in one and 5 nodes in the second. Mail.Ru’s ScyllaDB cluster handles 240,000 writes per second with 95% latency ~1.5ms and 99.9% latency ~22ms. It supports a peak of 100 reads per second with 95% latency ~400ms and 99.9% ~ 650ms.

ScyllaDB has enabled Mail.Ru to achieve the following results:

  • A high-load service for storing users actions with ScyllaDB and HDDs
  • 240K writes per second with 95% latency of 1.5ms
  • Reads are served via secondary keys with predictable performance

But why HDDs instead of SDDs? Kirill Alexseev, Software Engineering Technical Lead at Mail.Ru, explains: “By using HDDs, we saved at least $150,000 of capital expenses per petabyte compared to an SSD setup.”