Introduction

Uber uses MySQL as the underlying database engine for Schemaless and Docstore, our distributed databases. By default, MySQL uses the most popular InnoDB engine, a B+Tree structure for data storage. MyRocks is a MySQL storage engine that integrates with RocksDB, an open source project. The RocksDB store is based on the log-structured merge-tree (or LSM tree) and is optimized for fast storage and combines outstanding space and write efficiency with acceptable read performance. 

The Uber Storage Platform team has migrated all Schemaless instances and some Docstore instances to MyRocks since 2019. In this post, we are going to talk about the journey of our migration to MyRocks.

Motivation

Schemaless/Docstore are serving business-critical use cases across multiple lines of business and platforms e.g. rides, eats, grocery, maps, money movement etc. Both store tens of petabytes of data and serve tens of millions of requests/second. With the growth of Uber’s business, these numbers are only getting larger and the disk space has become the bottleneck: some nodes alone contain more than several terabytes, which makes it harder for some day to day operational tasks, such as data resync and backup. 

Disk usage was becoming the bottleneck resource and causing CPU and memory resources to be underutilized. This was leading to overall higher total cost of ownership (TCO) for running Schemaless. It was thus critical for us to address this issue and bend the cost curve.

Challenges

One way to address such an issue is splitting nodes to distribute disk space, however that is not only time consuming but also runs into the sharding limit on some instances, meaning node splitting is not a long term sustainable solution for us given our scale and growth. We tried to use InnoDB table compression to alleviate disk pressure, which gave us only about 5% disk space saving since data has already been compressed in our query engine layer before being stored in the database and additional compression will not yield much. 

While looking out for a solution, we did a benchmark with MyRocks, which achieved over 30% disk space saving with appropriate latency. One of the main reasons for this is that RocksDB does not use a fixed page size and hence can achieve efficient compression with least wastage. Also, InnoDB uses B+ tree index which can get fragmented leading to additional disk space usage. 

Essentially MyRocks is just MySQL with the RocksDB storage engine. It has the same client protocols and there is no change required in our query engine layer and clients, so we reached the conclusion that MyRocks would be the best choice to support Uber’s fast-growing business resulting in an increase of database footprint.

The migration of data and nodes at such a large scale while maintaining strict SLAs along with consistency, availability, and transparency is always difficult. The challenges include:

Upgrade and Migration

The topology of a Schemaless/Docstore instance has data divided into shards which are mapped to a configurable number of physical partitions. Each partition has multiple data copies within a single region and runs Raft consensus protocol for consistency. Each partition is also replicated across geographical regions.

To ensure the migration was transparent to the stakeholders without impact, we designed an incremental migration strategy, from single partition and to multiple partitions, from single region to all regions. Between each node migration, we validated the data integrity and monitored latency and error rate to make sure there was no performance degradation and all these steps were done automatically.

MySQL 8.0 Upgrade

Uber uses XtraBackup for data resync and backup. Xtrabackup started supporting MyRocks from the MySQL 8.0 version, so as the first step of the MyRocks migration process, we needed to upgrade our fleet to MySQL 8.0. Another motivation for the upgrade was because Docstore planned to use MySQL JSON features provided by MySQL 8.0.

We performed in-place MySQL upgrade with just binary replacement. The overall steps were as below:

It took several months to upgrade MySQL from 5.6 to 5.7 in 2018 at Uber, but from MySQL 5.7 to 8.0, it took over a year. We spent lots of time working around the bugs we hit in MySQL 8.0 and performance degradation, which included: 

MyRocks Migration

We simultaneously migrated MySQL to MyRocks on the nodes that have been upgraded to MySQL 8.0. Different from the MySQL 8.0 in-place upgrade, the MyRocks migration strategy was similar to the MySQL logical upgrade: we added the extra nodes in one region using Xtrabackup and started the migration process there.

The node migration included engine conversions on each table, and several validations to make sure the MyRocks node provides the same data consistency and query performance as InnoDB node.

If the first node was converted to MyRocks successfully in a partition and passed all the validations, we will continue with the rest of the nodes in the same partition. 

We repeated the same steps on all partitions in parallel in one region, and after it was fully converted to MyRocks, we paused the process and compared the performance between 2 regions (e.g., latency, error rate, CPU/IO utilization, etc.). We still could rollback and failover the traffic into the MySQL region if needed. 

If everything looked good in the MyRocks region, we could start the migration in another region. In this step, we would not add an extra MyRocks node, as everything has been verified in the previous region–we just replaced the MySQL node with MyRocks to save the extra capacity needed. Finally, we finished the MyRocks migration on an instance.

Lessons

By migrating to MyRocks we saw disk space savings of over 30% across the board. However, what we learned from the migration is that not all the use cases are good candidates that can be migrated to MyRocks. We observed higher CPU usage and increased disk IO utilization on some instances as a trade-off of disk space saving. 

The whole upgrade and migration took longer than the initial planned timeline. One of the reasons is MyRocks is still considered to be a very new database engine; there is no mature community from whom we can draw support, and we need to either wait for the patch from Percona or spend time to research and find our own workarounds whenever we run into issues. 

Next

It took several years to finish the Schemaless migration. We have also migrated some Docstore instances since then. Compared to Schemaless, we have seen much larger disk space savings with Docstore, since the data is not compressed before being stored. We plan to continue migrating Docstore to MyRocks.

We plan to focus on tuning MyRocks to ensure higher efficiency and performance for our dominant Schemaless and Docstore workloads. We also plan to solidify our overall backup recovery story for MyRocks.

If you are interested in working on large-scale distributed storage systems and taking on the challenge and responsibility of powering Uber’s products worldwide, please explore and apply to the open Storage team positions on Uber’s Careers page.

Header Image Attribution: The “Hourglass Dolphins in Drake Passage” image is covered by a CC BY-SA 3.0 license and is credited to Lomvi2.