850d759fd9
Consider a cluster with no data, e.g. in tests. When a new node is bootstrapped with repair we iterate over all (shard, table, range), read data from all the peer nodes for the range, look for any discrepancies and heal them. Even for small num_tokens (16 in the tests) the number of affected ranges (those we need to consider) amounts to total number of tokens in the cluster, which is 32 for the second node and 48 for the third. Multiplying this by the number of shards and the number of tables in each keyspace gives thousands of ranges. For each of them we need to follow some row level repair protocol, which includes several RPC exchanges between the peer nodes and creating some data structures on them. These exchanges are processed sequentially for each shard, there are `parallel_for_each` in code, but they are throttled by the choosen memory constraints and in fact execute sequentially.
When the bootstrapping node (master) reaches a peer node and asks for data in the specific range and master shard, two options exist. If sharder parameters (primarily, `--smp`) are the same on the master and on the peer, we can just read one local shard, this is fast. If, on the other hand, `--smp` is different, we need to do a multishard query. The given range from the master can contain data from different peer shards, so we split this range into a number of subranges such that each of them contain data only from the given master shard (`dht::selective_token_range_sharder`). The number of these subranges can be quite big (300 in the tests). For each of these subranges we do `fast_forward_to` on the `multishard_reader`, and this incurs a lot of overhead, mainly becuse of `smp::submit_to`.
In this series we optimize this case. Instead of splitting the master range and reading only what's needed, we read all the data in the range and then apply the filter by the master shard. We do this if the estimated number of partitions is small (<=100).
This is the logs of starting a second node with `--smp 4`, first node was `--smp 3`:
```
with this patch
20:58:49.644 INFO> [debug/topology_custom.test_topology_smp.1] starting server at host 127.222.46.3 in scylla-2...
20:59:22.713 INFO> [debug/topology_custom.test_topology_smp.1] started server at host 127.222.46.3 in scylla-2, pid 1132859
without this patch
21:04:06.424 INFO> [debug/topology_custom.test_topology_smp.1] starting server at host 127.181.31.3 in scylla-2...
21:06:01.287 INFO> [debug/topology_custom.test_topology_smp.1] started server at host 127.181.31.3 in scylla-2, pid 1134140
```
Fixes: #14093
Closes #14178
* github.com:scylladb/scylladb:
repair_test: add test_reader_with_different_strategies
repair: extract repair_reader declaration into reader.hh
repair_meta: get_estimated_partitions fix
repair_meta: use multishard_filter reader if the number of partitions is small
repair_meta: delay _repair_reader creation
database.hh: make_multishard_streaming_reader with range parameter
database.cc: extract streaming_reader_lifecycle_policy
Scylla
What is Scylla?
Scylla is the real-time big data database that is API-compatible with Apache Cassandra and Amazon DynamoDB. Scylla embraces a shared-nothing approach that increases throughput and storage capacity to realize order-of-magnitude performance improvements and reduce hardware costs.
For more information, please see the ScyllaDB web site.
Build Prerequisites
Scylla is fairly fussy about its build environment, requiring very recent versions of the C++20 compiler and of many libraries to build. The document HACKING.md includes detailed information on building and developing Scylla, but to get Scylla building quickly on (almost) any build machine, Scylla offers a frozen toolchain, This is a pre-configured Docker image which includes recent versions of all the required compilers, libraries and build tools. Using the frozen toolchain allows you to avoid changing anything in your build machine to meet Scylla's requirements - you just need to meet the frozen toolchain's prerequisites (mostly, Docker or Podman being available).
Building Scylla
Building Scylla with the frozen toolchain dbuild is as easy as:
$ git submodule update --init --force --recursive
$ ./tools/toolchain/dbuild ./configure.py
$ ./tools/toolchain/dbuild ninja build/release/scylla
For further information, please see:
- Developer documentation for more information on building Scylla.
- Build documentation on how to build Scylla binaries, tests, and packages.
- Docker image build documentation for information on how to build Docker images.
Running Scylla
To start Scylla server, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --workdir tmp --smp 1 --developer-mode 1
This will start a Scylla node with one CPU core allocated to it and data files stored in the tmp directory.
The --developer-mode is needed to disable the various checks Scylla performs at startup to ensure the machine is configured for maximum performance (not relevant on development workstations).
Please note that you need to run Scylla with dbuild if you built it with the frozen toolchain.
For more run options, run:
$ ./tools/toolchain/dbuild ./build/release/scylla --help
Testing
See test.py manual.
Scylla APIs and compatibility
By default, Scylla is compatible with Apache Cassandra and its APIs - CQL and Thrift. There is also support for the API of Amazon DynamoDB™, which needs to be enabled and configured in order to be used. For more information on how to enable the DynamoDB™ API in Scylla, and the current compatibility of this feature as well as Scylla-specific extensions, see Alternator and Getting started with Alternator.
Documentation
Documentation can be found here. Seastar documentation can be found here. User documentation can be found here.
Training
Training material and online courses can be found at Scylla University. The courses are free, self-paced and include hands-on examples. They cover a variety of topics including Scylla data modeling, administration, architecture, basic NoSQL concepts, using drivers for application development, Scylla setup, failover, compactions, multi-datacenters and how Scylla integrates with third-party applications.
Contributing to Scylla
If you want to report a bug or submit a pull request or a patch, please read the contribution guidelines.
If you are a developer working on Scylla, please read the developer guidelines.
Contact
- The community forum and Slack channel are for users to discuss configuration, management, and operations of the ScyllaDB open source.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.