0f59deffaa
When running those operations after a tablet replica is migrated away from
a shard, an assert can fail resulting in a crash.
Status quo (around the assert in truncate procedure):
1) Highest RP seen by table is saved in low_mark, and the current time in
low_mark_at.
2) Then compaction is disabled in order to not mix data written before truncate,
and data written later.
3) Then memtable is flushed in order for the data written before truncate to be
available in sstables and then removed.
4) Now, current time is saved in truncated_at, which is supposedly the time of
truncate to decide which sstables to remove.
Note: truncated_at is likely above low_mark_at due to steps 2 and 3.
The interesting part of the assert is:
(truncated_at <= low_mark_at ? rp <= low_mark : low_mark <= rp)
Note: RP in the assert above is the highest RP among all sstables generated
before truncated_at. RP is retrieved by table::discard_sstables().
If truncated_at > low_mark_at, maybe newer data was written during steps 2 and
3, and memtable's RP becomes greater than low_mark, resulting in a SSTable with
RP > low_mark.
So assert's 2nd condition is there to defend against the scenario above.
truncated_at and low_mark_at uses millisecond granularity, so even if
truncated_at == low_mark_at, data could have been written in steps 2 and 3
(during same MS window), failing the assert. This is fragile.
Reproducer:
To reproduce the problem, truncated_at must be > low_mark_at, which can easily
happen with both drop table and truncate due to steps 2 and 3.
If a shard has 2 or more tablets, the table's highest RP refer to just one
tablet in that shard.
If the tablet with the highest RP is migrated away, then the sstables in that
shard will have lower RP than the recorded highest RP (it's a table wide state,
which makes sense since CL is shared among tablets).
So when either drop table or truncate runs, low_mark will be potentially bigger
than highest RP retrieved from sstables.
Proposed solution:
The current assert is hacked to not fail if writes sneak in, during steps 2 and
3, but it's still fragile and seems not to serve its real purpose, since it's
allowing for RP > low_mark.
We should be able to say that low_mark >= RP, as a way of asserting we're not
leaving data targeted by truncate behind (or that we're not removing the wrong
data).
But the problem is that we're saving low_mark in step 1, before preparation
steps (2 and 3). When truncated_at is recorded in step 4, it's a way of saying
all data written so far is targeted for removal. But as of today, low_mark
refers to all data written up to step 1. So low_mark is now only one set
before issuing flush, and also accounts for all potentially flushed data.
Fixes #18059.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes scylladb/scylladb#23560
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++23 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 API - CQL. 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 ScyllaDB.
- The developers mailing list is for developers and people interested in following the development of ScyllaDB to discuss technical topics.