Make the removenode operation go through the `left_token_ring` state,
similar to decommission. This ensures that when removenode completes,
all nodes in the cluster are aware of the topology change through a
global token metadata barrier.
Previously, removenode would skip the `left_token_ring` state and go
directly from `write_both_read_new` to `left` state. This meant that
when the operation completed, some nodes might not yet know about the
topology change, potentially causing issues with subsequent data plane
requests.
Key changes:
- Both decommission and removenode now transition to `left_token_ring`
state in the `write_both_read_new` handler
- In `left_token_ring` state, only decommissioning nodes receive the
shutdown RPC (removed nodes may already be dead)
- Updated documentation to reflect that both operations use this state
This change improves consistency guarantees for removenode operations
by ensuring cluster-wide awareness before completion.
Fixes: scylladb/scylladb#25530
Previously, the view building coordinator relied on setting each task's state to STARTED and then explicitly removing these state entries once tasks finished, before scheduling new ones. This approach induced a significant number of group0 commits, particularly in large clusters with many nodes and tablets, negatively impacting performance and scalability.
With the update, the coordinator and worker logic has been restructured to operate without maintaining per-task states. Instead, tasks are simply tracked with an aborted boolean flag, which is still essential for certain tablet operations. This change removes much of the coordination complexity, simplifies the view building code, and reduces operational overhead.
In addition, the coordinator now batches reports of finished tasks before making commits. Rather than committing task completions individually, it aggregates them and reports in groups, significantly minimizing the frequency of group0 commits. This new approach is expected to improve efficiency and scalability during materialized view construction, especially in large deployments.
Fixes https://github.com/scylladb/scylladb/issues/26311
This patch needs to be backported to 2025.4.
Closesscylladb/scylladb#26897
* github.com:scylladb/scylladb:
docs/dev/view-building-coordinator: update the docs after recent changes
db/view/view_building: send coordinator's term in the RPC
db/view/view_building_state: replace task's state with `aborted` flag
db/view/view_building_coordinator: batch finished tasks reporting
db/view/view_building_worker: change internal implementation
db/view/view_building_coordinator: change `work_on_tasks` RPC return type
When migrating tablet size during the end_migration tablet transition stage, we need the pending and leaving replica hosts. The leaving and pending replicas are gathered in objects of type std::optional<tablet_replica> and are not checked if they contain a value before dereferencing which could cause an exception in the topology coordinator.
This patch adds a check for leaving and pending replicas, and only performs the tablet size migration if neither are empty.
This bug was introduced in 10f07fb95a
This change also adds the ability to create a tablet size in load_stats during end_migration stage of a tablet rebuild. We compute the new tablet size from by averaging the tablet sizes of the existing replicas.
This change also adds the virtual table tablet_sizes which contains tablet sizes of all the replicas of all the tablets in the cluster.
A version containing this bug has not yet been released, so a backport is not needed.
Closesscylladb/scylladb#27118
* github.com:scylladb/scylladb:
test: add tests for tablet size migration during end_migration
virtual_table: add tablet_sizes virtual table
load_stats: update tablet sizes after migration or rebuild
Add documentation for the quarantine/ subdirectory that holds SSTables
isolated due to validation failures or corruption. Document the scrub
operation's quarantine_mode parameter options and the drop_quarantined_sstables
API operation.
Also update the directory hierarchy example to include the quarantine directory.
Fixes#10742
Signed-off-by: Shreyas Ganesh <vansi.ganeshs@gmail.com>
Closesscylladb/scylladb#27023
Add a table name to Alternator's tracing output, as some clients would
like to consistently receive this information.
- add missing `tracing::add_table_name` in `executor::scan`
- add emiting tables' names in `trace_state::build_parameters_map`
- update tests, so when tracing is looked for it is filtered by table's
name, which confirms table is being outputed.
- change `struct one_session_records` declaration to `class one_session_records`,
as `one_session_records` is later defined as class.
Refs #26618Fixes#24031Closesscylladb/scylladb#26634
The updates include:
- adding missing parts like topology states and table rows,
- documenting zero-token nodes,
- replacing the old recovery procedure with the new one.
Fixes#26412
Updates of internal docs (usually read on master) don't require
backporting.
Closesscylladb/scylladb#27022
* github.com:scylladb/scylladb:
docs/dev/topology-over-raft: update the recovery section
docs/dev/topology-over-raft: document zero-token nodes
docs/dev/topology-over-raft: clarify the lack of tablet-specific states
docs/dev/topology-over-raft: add the missing join_group0 state
docs/dev/topology-over-raft: update the topology columns
Some of the columns were added, but the doc wasn't updated.
`upgrade_state` was updated in only one of the two places.
`ignore_nodes` was changed to a static column.
If the user connects to Scylla via the maintenance socket, it may happen
that `auth_integration` has not been registered in the service level
controller yet. One example is maintenance mode when that will never
happen; another when the connection occurs before Scylla is fully
initialized.
To avoid unnecessary crashes, we add new branches if the passed user is
absent or if it corresponds to the anonymous role. Since the role
corresponding to a connection via the maintenance socket is the anonymous
role, that solves the problem.
In those cases, we completely circumvent any calls to `auth_integration`
and handle them separately. The modified methods are:
* `get_user_scheduling_group`,
* `with_user_service_level`,
* `describe_service_levels`.
For the first two, the new behavior is in line with the previous
implementation of those functions. The last behaves differently now,
but since it's a soft error, crashing the node is not necessary anyway.
We throw an exception instead, whose error message should give the user
a hint of what might be wrong.
The other uses of `auth_integration` within the service level controller
are not problematic:
* `find_effective_service_level`,
* `find_cached_effective_service_level`.
They take the name of a role as their argument. Since the anonymous role
doesn't have a name, it's not possible to call them with it.
Fixesscylladb/scylladb#26816
In 380f243986 we added support for rack
lists in replication options. Drivers which are not prepared to parse
that (as of now, all of them), will not create metadata object for
that keyspace. This breaks, for example, the "copy to/from" cqlsh
command. Potentially other things too.
To fix that, keep the "replication" column in the old format, and
store numeric RF there, which corresponds to the number of
replicas. Accurate options in the new format are put in
"replication_v2".
We set replication_v2 in the schema only when it differs from the old
"replication" so that the new column is not set during upgrade,
otherwise downgrade would fail. Partition tombstone is added to ensure
that pre-alter replication_v2 value is deleted on alters which change
replication to a value which is the same as the post-alter
"replication" value.
Fixes#26415Closesscylladb/scylladb#26429
Augments the object storage document with config options etc for
using GS instead of S3.
TODO: add proper gsutil command line examples for manual managing of
GCP storage.
Allows per-DC replication factor to be either a string, holding a
numerical value, or a list of strings, holding a list of rack names.
The rack list is not respected yet by the tablet allocator, this is
achieved in subsequent commit.
This changes the format of options stored in the flattened map
in system_schema.keyspaces#replication. Values which are rack lists,
are converted into multiple entries, with the list index appended to
the key with ':' as the separator:
For example, this extended map:
{
'dc1': '3',
'dc2': ['rack1', 'rack2']
}
is stored as a flattened map:
{
'dc1': '3',
'dc2:0': 'rack1',
'dc2:1': 'rack2'
}
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@scylladb.com>
Introduce `ms` -- a new sstable format version which
is a hybrid of Cassandra's `me` and `da`.
It is based on `me`, but with the index components
(Summary.db and Index.db) replaced with the index
components of `da` (Partitions.db and Rows.db).
As of this patch, the version is never chosen
anywhere for writing sstables yet. It is only introduced.
We will add it to unit tests in a later commit,
and expose it to users in yet later commit.
initial implementation to support CDC in tablets-enabled keyspaces.
The design is described in https://docs.google.com/document/d/1qO5f2q5QoN5z1-rYOQFu6tqVLD3Ha6pphXKEqbtSNiU/edit?usp=sharing
It is followed closely for the most part except "Deciding when to change streams" - instead, streams are changed synchronously with tablet split / merge.
Instead of the stream switching algorithm with the double writes, we use a scheme similar to the previous method for vnodes - we add the new streams with timestamp that is sufficiently far into the future.
In this PR we:
* add new group0-based internal system tables for tablet stream metadata and loading it into in-memory CDC metadata
* add virtual tables for CDC consumers
* the write coordinator chooses a stream by looking up the appropriate stream in the CDC metadata
* enable creating tables with CDC enabled in tablets-enabled keyspaces. tablets are allocated for the CDC table, and a stream is created per each tablet.
* on tablet resize (split / merge), the topology coordinator creates a new stream set with a new stream for each new tablet.
* the cdc tablets are co-located with the base tablets
Fixes https://github.com/scylladb/scylladb/issues/22576
backport not needed - new feature
update dtests: https://github.com/scylladb/scylla-dtest/pull/5897
update java cdc library: https://github.com/scylladb/scylla-cdc-java/pull/102
update rust cdc library: https://github.com/scylladb/scylla-cdc-rust/pull/136Closesscylladb/scylladb#23795
* github.com:scylladb/scylladb:
docs/dev: update CDC dev docs for tablets
doc: update CDC docs for tablets
test: cluster_events: enable add_cdc and drop_cdc
test/cql: enable cql cdc tests to run with tablets
test: test_cdc_with_alter: adjust for cdc with tablets
test/cqlpy: adjust cdc tests for tablets
test/cluster/test_cdc_with_tablets: introduce cdc with tablets tests
cdc: enable cdc with tablets
topology coordinator: change streams on tablet split/merge
cdc: virtual tables for cdc with tablets
cdc: generate_stream_diff helper function
cdc: choose stream in tablets enabled keyspaces
cdc: rename get_stream to get_vnode_stream
cdc: load tablet streams metadata from tables
cdc: helper functions for reading metadata from tables
cdc: colocate cdc table with base
cdc: remove streams when dropping CDC table
cdc: create streams when allocating tablets
migration_listener: add on_before_allocate_tablet_map notification
cdc: notify when creating or dropping cdc table
cdc: move cdc table creation to pre_create
cdc: add internal tables for cdc with tablets
cdc: add cdc_with_tablets feature flag
cdc: add is_log_schema helper
According to the changes in Vector Store API (VECTOR-148) the `embedding` term
should be changed to `vector`. As `vector` term is used for STL class the
internal type or variable names would be changed to `vs_vector` (aka vector
store vector). This patch changes also the HTTP ann json request payload
according to the Vector Store API changes.
Fixes: VECTOR-229
Closesscylladb/scylladb#26050
This patch introduces a new `incremental_mode` parameter to the tablet
repair REST API, providing more fine-grained control over the
incremental repair process.
Previously, incremental repair was on and could not be turned off. This
change allows users to select from three distinct modes:
- `regular`: This is the default mode. It performs a standard
incremental repair, processing only unrepaired sstables and skipping
those that are already repaired. The repair state (`repaired_at`,
`sstables_repaired_at`) is updated.
- `full`: This mode forces the repair to process all sstables, including
those that have been previously repaired. This is useful when a full
data validation is needed without disabling the incremental repair
feature. The repair state is updated.
- `disabled`: This mode completely disables the incremental repair logic
for the current repair operation. It behaves like a classic
(pre-incremental) repair, and it does not update any incremental
repair state (`repaired_at` in sstables or `sstables_repaired_at` in
the system.tablets table).
The implementation includes:
- Adding the `incremental_mode` parameter to the
`/storage_service/repair/tablet` API endpoint.
- Updating the internal repair logic to handle the different modes.
- Adding a new test case to verify the behavior of each mode.
- Updating the API documentation and developer documentation.
Fixes#25605Closesscylladb/scylladb#25693
add --dc and --rack commandline arguments to the scylla docker image, to
allow starting a node with a specified dc and rack names in a simple
way.
This is useful mostly for small examples and demonstrations of starting
multiple nodes with different racks, when we prefer not to bother with
editing configuration files. The ability to assign nodes to different
racks is especially important with RF=Rack enforcing.
The previous method to achieve this is to set the snitch to
GossipingPropertyFileSnitch and provide a configuration file in
/etc/scylla/cassandra-rackdc.properties with the name of the dc and
rack.
The new dc and rack parameters are implemented similarly by using the
snitch GossipingPropertyFileSnitch and writing the dc and rack values to
the rackdc properties file. We don't support passing the parameters
together with a different snitch, or when mounting a properties file
from the host, because we don't want to overwrite it.
Example:
docker run -d --name scylla1 scylladb/scylla --dc my_dc1 --rack my_rack1
Fixesscylladb/scylladb#23423Closesscylladb/scylladb#25607
As part of removing redis from Scylla source tree.
This commit removes all related documentation.
Following commit remove the code itself.
Signed-off-by: Ran Regev <ran.regev@scylladb.com>
The central idea of incremental repair is to allow repair participants
to select and repair only a portion of the dataset to speed up the
repair process. All repair participants must utilize an identical
selection method to repair and synchronize the same selected dataset.
There are two primary selection methods: time-based and file-based. The
time-based method selects data within a specified time frame. It is
versatile but it is less efficient because it requires reading all of
the dataset and omitting data beyond the time frame. The file-based
method selects data from unrepaired SSTables and is more efficient
because it allows the entire SSTable to be omitted. This document patch
implements the file-based selection method.
Incremental repair will only be supported for tablet tables; it will not
be supported for vnode tables. On one hand, the legacy vnode is less
important to support. On the other hand, the incremental repair for
vnode is much harder to implement. With vnodes, a SSTalbe could contain
data for multiple vnode ranges. When a given vnode range is repaired,
only a portion of the SSTable is repaired. This complicates the
manipulation of SSTables significantly during both repair and
compaction. With tablets, an entire tablet is repaired so that a
sstable is either fully repaired or not repaired which is a huge
simplification.
This patch uses the repaired_at from sstables::statistics component to
mark a sstable as repaired. It uses a virtual clock as the repair
timestamp, i.e., using a monotonically increasing number for the
repaired_at field of a SSTable and sstables_repaired_at column in
system.tablets table. Notice that when a sstable is not repaired, the
repaired_at field will be set to the default value 0 by default. The
being_repaired in memory field of a SSTable is used to explicitly mark
that a SSTable is being selected. The following variables are used for
incremental repair:
The repaired_at on disk field of a SSTable is used.
- A 64-bit number increases sequentially
The sstables_repaired_at is added to the system.tablets table.
- repaired_at <= sstables_repaired_at means the sstable is repaired
The being_repaired in memory field of a SSTable is added.
- A repair UUID tells which sstable has participated in the repair
Initial test results:
1) Medium dataset results
Node amount: 3
Instance type: i4i.2xlarge
Disk usage per node: ~500GB
Cluster pre-populated with ~500GB of data before starting repairs job.
Results for Repair Timings:
The regular repair run took 210 mins.
Incremental repair 1st run took 183 mins, 2nd and 3rd runs took around 48s
The speedup is: 183 mins / 48s = 228X
2) Small dataset results
Node amount: 3
Instance type: i4i.2xlarge
Disk usage per node: ~167GB
Cluster pre-populated with ~167GB of data before starting the repairs job.
Regular repair 1st run took 110s, 2nd and 3rd runs took 110s.
Incremental repair 1st run took 110 seconds, 2nd and 3rd run took 1.5 seconds.
The speedup is: 110s / 1.5s = 73X
3) Large dataset results
Node amount: 6
Instance type: i4i.2xlarge, 3 racks
50% of base load, 50% read/write
Dataset == Sum of data on each node
Dataset Non-incremental repair (minutes)
1.3 TiB 31:07
3.5 TiB 25:10
5.0 TiB 19:03
6.3 TiB 31:42
Dataset Incremental repair (minutes)
1.3 TiB 24:32
3.0 TiB 13:06
4.0 TiB 5:23
4.8 TiB 7:14
5.6 TiB 3:58
6.3 TiB 7:33
7.0 TiB 6:55
Fixes#22472Closesscylladb/scylladb#24291
* github.com:scylladb/scylladb:
replica: Introduce get_compaction_reenablers_and_lock_holders_for_repair
compaction: Move compaction_reenabler to compaction_reenabler.hh
topology_coordinator: Make rpc::remote_verb_error to warning level
repair: Add metrics for sstable bytes read and skipped from sstables
test.py: Disable incremental for test_tombstone_gc_for_streaming_and_repair
test.py: Add tests for tablet incremental repair
repair: Add tablet incremental repair support
compaction: Add tablet incremental repair support
feature_service: Add TABLET_INCREMENTAL_REPAIR feature
tablet_allocator: Add tablet_force_tablet_count_increase and decrease
repair: Add incremental helpers
sstable: Add being_repaired to sstable
sstables: Add set_repaired_at to metadata_collector
mutation_compactor: Introduce add operator to compaction_stats
tablet: Add sstables_repaired_at to system.tablets table
test: Fix drain api in task_manager_client.py
Documentation had outdated information how to run C++ test.
Additionally, some information added about gathered test metrics.
Closesscylladb/scylladb#25180
This patch is a part of vector_store_client sharded service
implementation for a communication with vector-store service.
It implements a functionality for ANN search request to a vector-store
service. It sends request, receive response and after parsing it returns
the list of primary keys.
It adds json parsing functionality specific for the HTTP ANN API.
It adds a hardcoded http request timeout for retrieving response from
the Vector Store service.
It also adds an automatic boost test of the ANN search interface, which
uses a mockup http server in a background to simulate vector-store
service.
It adds a documentation for HTTP API protocol used used for ANN
functionality.
Fixes: VS-47
This PR introduces a new `comparable_bytes` class to add byte-comparable format support for all the [native cql3 data types](https://opensource.docs.scylladb.com/stable/cql/types.html#native-types) except `counter` type as that is not comparable. The byte-comparable format is a pre-requisite for implementing the trie based index format for our sstables(https://github.com/scylladb/scylladb/issues/19191). This implementation adheres to the byte-comparable format specification in https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/utils/bytecomparable/ByteComparable.md
Note that support for composite data types like lists, maps, and sets has not been implemented yet and will be made available in a separate PR.
Refs https://github.com/scylladb/scylladb/issues/19407
New feature - backport not required.
Closesscylladb/scylladb#23541
* github.com:scylladb/scylladb:
types/comparable_bytes: add testcase to verify compatibility with cassandra
types/comparable_bytes: support variable-length natively byte-ordered data types
types/comparable_bytes: support decimal cql3 types
types/comparable_bytes: introduce count_digits() method
types/comparable_bytes: support uuid and timeuuid cql3 types
types/comparable_bytes: support varint cql3 type
types/comparable_bytes: support skipping sign byte write in decode_signed_long_type
types/comparable_bytes: introduce encode/decode_varint_length
types/comparable_bytes: support float and double cql3 types
types/comparable_bytes: support date, time and timestamp cql3 types
types/comparable_bytes: support bigint cql3 type
types/comparable_bytes: support fixed length signed integers
types/comparable_bytes: support boolean cql3 type
types: introduce comparable_bytes class
bytes_ostream: overload write() to support writing from FragmentedView
docs: fix minor typo in docs/dev/cql3-type-mapping.md
Add a new column base_table to the system.tablets table.
It can be set to point to another table to indicate that the tablets of
this table are co-located with the tablets of the base table.
When it's set, we don't store other tablet information in system.tablets
and in the in-memory tablet map object for this table, and we need to
refer instead to the base table tablet information. The method
get_tablet_map always returns the base tablet map.
Although valid for compact tables, non-full (or empty) clustering key prefixes are not handled for row keys when writing sstables. Only the present components are written, consequently if the key is empty, it is omitted entirely.
When parsing sstables, the parsing code unconditionally parses a full prefix.
This mis-match results in parsing failures, as the parser parses part of the row content as a key resulting in a garbage key and subsequent mis-parsing of the row content and maybe even subsequent partitions.
Introduce a new system table: `system.corrupt_data` and infrastructure similar to `large_data_handler`: `corrupt_data_handler` which abstracts how corrupt data is handled. The sstable writer now passes rows such corrupt keys to the corrupt data handler. This way, we avoid corrupting the sstables beyond parsing and the rows are also kept around in system.corrupt_data for later inspection and possible recovery.
Add a full-stack test which checks that rows with bad keys are correctly handled.
Fixes: https://github.com/scylladb/scylladb/issues/24489
The bug is present in all versions, has to be backported to all supported versions.
Closesscylladb/scylladb#24492
* github.com:scylladb/scylladb:
test/boost/sstable_datafile_test: add test for corrupt data
sstables/mx/writer: handler rows with empty keys
test/lib/cql_assertions: introduce columns_assertions
sstables: add corrupt_data_handler to sstables::sstables
tools/scylla-sstable: make large_data_handler a local
db: introduce corrupt_data_handler
mutation: introduce frozen_mutation_fragment_v2
mutation/mutation_partition_view: read_{clustering,static}_row(): return row type
mutation/mutation_partition_view: extract de-ser of {clustering,static} row
idl-compiler.py: generate skip() definition for enums serializers
idl: extract full_position.idl from position_in_partition.idl
db/system_keyspace: add apply_mutation()
db/system_keyspace: introduce the corrupt_data table
This change adds an md file which gives a high
level overview of the scylladb repository, the
components each path contains and a basic description
for each one of them. This is mainly intended for
onboarding engineers to help get a mental picture when
starting ramping up on Scylla concepts.
Refs #22908
Signed-off-by: Robert Bindar <robert.bindar@scylladb.com>
Closesscylladb/scylladb#23010
To serve as a place to store corrupt mutation fragments. These fragments
cannot be written to sstables, as they would be spread around by
compaction and/or repair. They even might make parsing the sstable
impossible. So they are stored in this special table instead, kept
around to be inspected later and possibly restored if possible.
Currently only one global topology request (such as truncate, cdc repair, cleanup and alter table) can be pending. If one is already pending others will be rejected with an error. This is not very user friendly, so this series introduces a queue of global requests which allows queuing many global topology requests simultaneously.
Fixes: #16822
No need to backport since this is a new feature.
Closesscylladb/scylladb#24293
* https://github.com/scylladb/scylladb:
topology coordinator: simplify truncate handling in case request queue feature is disable
topology coordinator: fix indentation after the previous patch
topology coordinator: allow running multiple global commands in parallel
topology coordinator: Implement global topology request queue
topology coordinator: Do not cancel global requests in cancel_all_requests
topology coordinator: store request type for each global command
topology request: make it possible to hold global request types in request_type field
topology coordinator: move alter table global request parameters into topology_request table
topology coordinator: move cleanup global command to report completion through topology_request table
topology coordinator: no need to create updates vector explicitly
topology coordinator: use topology_request_tracking_mutation_builder::done() instead of open code it
topology coordinator: handle error during new_cdc_generation command processing
topology coordinator: remove unneeded semicolon
topology coordinator: fix indentation after the last commit
topology coordinator: move new_cdc_generation topology request to use topology_request table for completion
gms/feature_service: add TOPOLOGY_GLOBAL_REQUEST_QUEUE feature flag
This patch intends to give an overview of where, when and how we store
data in S3 and provide a quick set of commands
which help gain local access to the data in case there is a need for
manual intervention.
The patch also collects in the same place links/descriptions for all
formats we use in S3.
Fixes#22438
Signed-off-by: Robert Bindar <robert.bindar@scylladb.com>
Closesscylladb/scylladb#24323
Requests, together with their parameters, are added to the
topology_request tables and the queue of active global requests is
kept in topology state. Thy are processed one by one by the topology
state machine.
Fixes: #16822
Create a custom pytest test collector for .cql files and
move CQL test execution logic from `CQLApprovalTest` class
and `pylib/cql_repl/cql_repl.py` file to `CqlTest.runtest()`
method.
In result, the only difference between CQLApproval and Python
suite types is suffixes of test files.
Metadata id was introduced in CQLv5 to make metadata of prepared
statement consistent between driver and database. This commit introduces
a protocol extension that allows to use the same mechanism in CQLv4.
This change:
- Introduce SCYLLA_USE_METADATA_ID protocol extension for CQLv4
- Introduce METADATA_CHANGED flag in RESULT. The flag cames directly
from CQLv5 binary protocol. In CQLv4, the bit was never used, so we
assume it is safe to reuse it.
- Implement handling of metadata_id and METADATA_CHANGED in RESULT rows
- Implement returning metadata_id in RESULT prepared
- Implement reading metadata_id from EXECUTE
- Added description of SCYLLA_USE_METADATA_ID in documentation
Metadata_id is wrapped in cql_metadata_id_wrapper because we need to
distinguish the following situations:
- Metadata_id is not supported by the protocol (e.g. CQLv4 without the
extension is used)
- Metadata_id is supported by the protocol but not set - e.g. PREPARE
query is being handled: it doesn't contain metadata_id in the
request but the reply (RESULT prepared) must contain metadata_id
- Metadata_id is supported by the protocol and set, any number of
bytes >= 0 is allowed, according to the CQLv5 protocol specification
Fixesscylladb/scylladb#20860
Documentation for --gather-metric parameter was missing. This functionality can
break regular flow of using test.py, because of possible misconfiguration of
the cgroup on the local machine. Added explanation how to deal with potential
issue of gathering metrics functionality and how to switch it off.
Fixes: https://github.com/scylladb/scylladb/issues/20763Closesscylladb/scylladb#24095
