c5668d99c9
Unlike with vnodes, each tablet is served only by a single shard, and it is associated with a memtable that, when flushed, it creates sstables which token-range is confined to the tablet owning them. On one hand, this allows for far better agility and elasticity since migration of tablets between nodes or shards does not require rewriting most if not all of the sstables, as required with vnodes (at the cleanup phase). Having too few tablets might limit performance due not being served by all shards or by imbalance between shards caused by quantization. The number of tabelts per table has to be a power of 2 with the current design, and when divided by the number of shards, some shards will serve N tablets, while others may serve N+1, and when N is small N+1/N may be significantly larger than 1. For example, with N=1, some shards will serve 2 tablet replicas and some will serve only 1, causing an imbalance of 100%. Now, simply allocating a lot more tablets for each table may theoretically address this problem, but practically: a. Each tablet has memory overhead and having too many tablets in the system with many tables and many tablets for each of them may overwhelm the system's and cause out-of-memory errors. b. Too-small tablets cause a proliferation of small sstables that are less efficient to acces, have higher metadata overhead (due to per-sstable overhead), and might exhaust the system's open file-descriptors limitations. The options introduced in this change can help the user tune the system in two ways: 1. Sizing the table to prevent unnecessary tablet splits and migrations. This can be done when the table is created, or later on, using ALTER TABLE. 2. Controlling min_per_shard_tablet_count to improve tablet balancing, for hot tables. Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Scylla unit tests using C++ and the Boost test framework
The source files in this directory are Scylla unit tests written in C++ using the Boost.Test framework. These unit tests come in three flavors:
-
Some simple tests that check stand-alone C++ functions or classes use Boost's
BOOST_AUTO_TEST_CASE. -
Some tests require Seastar features, and need to be declared with Seastar's extensions to Boost.Test, namely
SEASTAR_TEST_CASE. -
Even more elaborate tests require not just a functioning Seastar environment but also a complete (or partial) Scylla environment. Those tests use the
do_with_cql_env()ordo_with_cql_env_thread()function to set up a mostly-functioning environment behaving like a single-node Scylla, in which the test can run.
While we have many tests of the third flavor, writing new tests of this type should be reserved to white box tests - tests where it is necessary to inspect or control Scylla internals that do not have user-facing APIs such as CQL. In contrast, black-box tests - tests that can be written only using user-facing APIs, should be written in one of newer test frameworks that we offer - such as test/cqlpy or test/alternator (in Python, using the CQL or DynamoDB APIs respectively) or test/cql (using textual CQL commands), or - if more than one Scylla node is needed for a test - using the test/topology* framework.
Running tests
Because these are C++ tests, they need to be compiled before running.
To compile a single test executable row_cache_test, use a command like
ninja build/dev/test/boost/row_cache_test
You can also use ninja dev-test to build all C++ tests, or use
ninja deb-build to build the C++ tests and also the full Scylla executable
(however, note that full Scylla executable isn't needed to run Boost tests).
Replace "dev" by "debug" or "release" in the examples above and below to use the "debug" build mode (which, importantly, compiles the test with ASAN and UBSAN enabling on and helps catch difficult-to-catch use-after-free bugs) or the "release" build mode (optimized for run speed).
To run an entire test file row_cache_test, including all its test
functions, use a command like:
build/dev/test/boost/row_cache_test -- -c1 -m1G
to run a single test function test_reproduce_18045() from the longer test
file, use a command like:
build/dev/test/boost/row_cache_test -t test_reproduce_18045 -- -c1 -m1G
In these command lines, the parameters before the -- are passed to
Boost.Test, while the parameters after the -- are passed to the test code,
and in particular to Seastar. In this example Seastar is asked to run on one
CPU (-c1) and use 1G of memory (-m1G) instead of hogging the entire
machine. The Boost.Test option -t test_reproduce_18045 asks it to run just
this one test function instead of all the test functions in the executable.
Unfortunately, interrupting a running test with control-C while doesn't
work. This is a known bug (#5696). Kill a test with SIGKILL (-9) if you
need to kill it while it's running.
Boost tests can also be run using test.py - which is a script that provides a uniform way to run all tests in scylladb.git - C++ tests, Python tests, etc.
Writing tests
Because of the large build time and build size of each separate test executable, it is recommended to put test functions into relatively large source files. But not too large - to keep compilation time of a single source file (during development) at reasonable levels.
When adding new source files in test/boost, don't forget to list the new source file in configure.py and also in CMakeLists.txt. The former is needed by our CI, but the latter is preferred by some developers.