Saturday, April 23, 2022

Variable sized disk rows in RonDB

 RonDB was a pure in-memory database engine in its origin. The main reason for this was to support low latency applications in the telecom business. However already in 2005 we presented a design at VLDB in Trondheim for the introduction of columns stored on disk. These columns cannot be indexed, but is very suitable for columns with large sizes.

RonDB is currently targeting Feature Store applications. These applications often access data through a set of primary key lookups where each row can have hundreds of columns with varying size.

In RonDB 21.04 the support for disk columns uses a fixed size disk row. This works very well to support handling small files in HopsFS. HopsFS is a distributed file system that can handle petabytes of storage in an efficient manner. On top of it Hopsworks build the offline Feature Store applications.

The small files are stored in a set of fixed size rows in RonDB with suitable sizes. YCSB benchmarks have shown that RonDB can handle writes of up to several GBytes per second. Thus the disk implementation of RonDB is very efficient.

Applications using the online Feature Store will however store much of its data in variable sized columns. These work perfectly well in the in-memory columns. They work also in the disk columns in RonDB 21.04. However to make storage more efficient we are designing a new version of RonDB where the row parts on disk are stored on variable sized disk pages.

These pages use the same data structure as the in-memory variable sized pages. So the new format only affects handling free space, handling of recovery. This design has now reached a state where it is passing our functional test suites. We will still add more tests, perform system tests and search for even more problems before we release for production usage.

One interesting challenge that can happen with a variable sized rows is that one might have to use more space in a data page. If this space isn't available we have to find a new page where space is available. It becomes an interesting challenge when taking into account that we can abort operations on a row while still committing other operations on the same row. The conclusion here is that one can never release any allocated resources until you fully commit or fully abort the transaction.

This type of challenge is one reason why it is so interesting to work with the internals of a distributed database engine. After 30 years of education, development and support, there are still new interesting challenges to handle.

Another challenge we faced was that we need to page in multiple data pages to handle an operation on the row. This means that we have to ensure that while paging in one data page, that other pages that we already paged in won't be paged out before we have completed our work on the row. This work also prepares the stage for handling rows that span over multiple disk pages. RonDB already supports rows that span multiple in-memory pages and one disk page.

If you want to learn more about RonDB requirements, LATS properties, use cases and internal algorithms, join us on Monday CMU Vaccin database presentation. Managed RonDB is supported on AWS, Azure and GCP and on-prem.

If you like to join the effort to develop RonDB and a managed RonDB version we have open positions at Hopsworks AB. Contact me at LinkedIn if you are interested.

Monday, January 31, 2022

RonDB receives ARM64 support and large transaction support

 RonDB is the base platform for all applications in Hopsworks. Hopsworks is a machine learning platform featuring a Feature Store that can be used in online applications as well as offline applications.

This means that RonDB development is driven towards ensuring that operating RonDB in this environment is the best possible.

RonDB is designed for millions of small transactions reading and writing data. However occasionally applications perform rather large transactions. Previous versions of RonDB had some weaknesses in this area. The new versions of RonDB now supports also large transactions although the focus is still on many smaller transactions.

Designing this new support of large transactions required a fairly large development effort. To do this in a stable release is a challenge, therefore it was decided to combine this effort with a heavy testing period focused on fixing bugs.

This effort has been focused on achieving three objectives. First to stabilise the new RonDB 21.04 releases which is the stable release of RonDB. Second, to stabilise the next RonDB release at the same level as RonDB 21.04. Third, we also wanted the same level of support for ARM64 machines.

We are now proud to release RonDB 21.04.3, a new stable release of RonDB that supports much larger transactions. Since the release of RonDB 21.04.1 in July 2021 we have fixed more than 50 bugs in RonDB and we are very satisfied with the stability also on ARM64 machines.

The original plan was to release the next version of RonDB in October 2021, however we didn't want to release a new version with any less stability than the RonDB 21.04 release. Thus instead we release this new version of RonDB now, RonDB 22.01.0.

ARM64 support covers both RonDB 21.04.3 and RonDB 22.01.0. RonDB is now also supported on both Linux and Mac OS X and on Windows it is supported using WSL 2 (Linux on Windows) on Windows 11. We have extensively tested RonDB on the following platforms:

  1. Mac OS X 11.6 x86_64
  2. Mac OS X 12.2 ARM64
  3. Windows WSL 2 Ubuntu x86_64
  4. Ubuntu 21.04 x86_64
  5. Oracle Linux 8 Cloud Developer version ARM64

It is used in production on AWS and Azure and has been extensively tested also on GCP and Oracle Cloud.

As part of the new RonDB release we have also updated the documentation of RonDB at docs.rondb.com. Among other things it contains a new section on Contributing to RonDB that shows how you can build, test and develop extensions to RonDB. In the documentation you will also find an extensive list of the improvements made in the two new RonDB releases.

ARM64 support is still in beta phase, our plan is to make it available for production use in Q2 2022. There are no known bugs, but we want to give it a bit more time before we assign it to production workloads. This includes adding more test machines and also performing benchmarks on ARM64 VMs.

Our experience with ARM64 machines so far says that it is fairly stable, but it isn't yet at the same level as x86, it is possible to find bugs in the compilers, the support around it is however maturing very quickly and not surprising the support on Mac OS X is here leading the way since Mac OS X has fully committed its future on ARM. We have also great help of participating in the OCI ARM Accelerator program providing access to ARM VMs in the Oracle Cloud making it possible to test on Oracle Linux using ARM with both small and large VMs.

RonDB 22.01.0 comes with a set of new features:

  1. Now possible to scale reads using locks onto more threads
  2. Improved placement of primary replicas to enable
  3. All major memory areas now managed by global memory manager
  4. Even more flexibility in thread configurations
  5. Removing a scalability hog in index statistics handling
  6. Merged with MySQL Cluster 8.0.28

You can either download RonDB tarballs from https://github.com/logicalclocks/rondb or from https://repo.hops.works/master, for exact links to the various versions of the binary tarballs see Release Notes on each version.