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This PR introduces the vnodes-to-tablets migration procedure, which enables converting an existing vnode-based keyspace to tablets.
The migration is implemented as a manual, operator-driven process executed in several stages. The core idea is to first create tablet maps with the same token boundaries and replica hosts as the vnodes, and then incrementally convert the storage of each node to the tablets layout. At a high level, the procedure is the following:
1. Create tablet maps for all tables in the keyspace.
2. Sequentially upgrade all nodes from vnodes to tablets:
1. Mark a node for upgrade in the topology state.
2. Restart the node. During startup, while the node is offline, it reshards the SSTables on vnode boundaries and switches to a tablet ERM.
3. Wait for the node to return online before proceeding to the next node.
4. Finalize the migration:
1. Update the keyspace schema to mark it as tablet-based.
2. Clear the group0 state related to the migration.
From the client's perspective, the migration is online; the cluster can still serve requests on that keyspace, although performance may be temporarily degraded.
During the migration, some nodes use vnode ERMs while others use tablet ERMs. Cluster-level algorithms such as load balancing will treat the keyspace's tables as vnode-based. Once migration is finalized, the keyspace is permanently switched to tablets and cannot be reverted back to vnodes. However, a rollback procedure is available before finalization.
The patch series consists of:
* Load balancer adjustments to ignore tablets belonging to a migrating keyspace.
* A new vnode-based resharding mode, where SSTables are segregated on vnode boundaries rather than with the static sharder.
* A new per-node `intended_storage_mode` column in `system.topology`. Represents migration intent (whether migration should occur on restart) and direction.
* Four new REST endpoints for driving the migration (start, node upgrade/downgrade, finalize, status), along with `nodetool` wrappers. The finalization is implemented as a global topology request.
* Wiring of the migration process into the startup logic: the `distributed_loader` determines a migrating table's ERM flavor from the `intended_storage_mode` and the ERM flavor determines the `table_populator`'s resharding mode. Token metadata changes have been adjusted to preserve the ERM flavor.
* Cluster tests for the migration process.
Fixes SCYLLADB-722.
Fixes SCYLLADB-723.
Fixes SCYLLADB-725.
Fixes SCYLLADB-779.
Fixes SCYLLADB-948.
New feature, no backport is needed.
Closes scylladb/scylladb#29065
* github.com:scylladb/scylladb:
docs: Add ops guide for vnodes-to-tablets migration
test: cluster: Add test for migration of multiple keyspaces
test: cluster: Add test for error conditions
test: cluster: Add vnodes->tablets migration test (rollback)
test: cluster: Add vnodes->tablets migration test (1 table, 3 nodes)
test: cluster: Add vnodes->tablets migration test (1 table, 1 node)
scylla-nodetool: Add migrate-to-tablets subcommand
api: Add REST endpoint for vnode-to-tablet migration status
api: Add REST endpoint for migration finalization
topology_coordinator: Add `finalize_migration` request
database: Construct migrating tables with tablet ERMs
api: Add REST endpoint for upgrading nodes to tablets
api: Add REST endpoint for starting vnodes-to-tablets migration
topology_state_machine: Add intended_storage_mode to system.topology
distributed_loader: Wire vnode-based resharding into table populator
replica: Pick any compaction group for resharding
compaction: resharding_compaction: add vnodes_resharding option
storage_service: Preserve ERM flavor of migrating tables
tablet_allocator: Exclude migrating tables from load balancing
feature_service: Add vnodes_to_tablets_migrations feature
Scylla developer documentation
This folder contains developer-oriented documentation concerning the ScyllaDB codebase. We also have a wiki, which contains additional developer-oriented documentation. There is currently no clear definition of what goes where, so when looking for something be sure to check both.
Seastar documentation can be found here.
User documentation can be found on docs.scylladb.com
For information on how to build Scylla and how to contribute visit HACKING.md and CONTRIBUTING.md.