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locator: tablets: Distribute data evenly among primary replicas during restore

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Most likely 817fdad uncovered the fact that our choice of
primary replica was resonating with tablet allocation and we were ending up
picking the same replica as primary within a scope instead of rotating
primaryship among all replicas in the scope.
This created situations where for instance, restoring into a 9 nodes cluster
with primary_replica_only=true would put all data into 3 nodes, leaving
the other 6 unused. The balancing of the dataset was performed by the
subsequent repair step.

split from bhalevy/load-balance-primary-replica

Fixes #27281

Signed-off-by: Robert Bindar <robert.bindar@scylladb.com>
This commit is contained in:
Robert Bindar
2025-12-06 09:11:32 +02:00
parent 92c988514c
commit d88036db48
1 changed files with 17 additions and 1 deletions
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18
locator/tablets.cc
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@@ -576,7 +576,23 @@ std::optional<tablet_replica> maybe_get_primary_replica(tablet_id id, const tabl
tablet_replica_set replica_set_copy = replica_set;
std::ranges::sort(replica_set_copy, tablet_replica_comparator(topo));
const auto replicas = replica_set_copy | std::views::filter(std::move(filter)) | std::ranges::to<tablet_replica_set>();
return !replicas.empty() ? std::make_optional(replicas.at(size_t(id) % replicas.size())) : std::nullopt;
if (replicas.empty()) {
tablet_logger.debug("No replicas in scope for tablet {}", id);
return std::nullopt;
}
// Once the filter was pushed down, we got here a set of replicas which live within the selected scope.
// replicas[ (id + id / size) % size ] is used to distribute the load evenly across all replicas in the scope.
// Let's take for instance a cluster with 1 dc, 3 racks, 9 nodes, rf=3, scope=dc. The `replicas` array here will contain 3
// replicas - one from each rack.
// Tablet id=0 will end up choosing replicas[0]
// Tablet id=1 will end up choosing replicas[1]
// Tablet id=2 will end up choosing replicas[2]
// We want tablet id=3 to choose replicas[1] and subsequently tablet id=6 to pick replicas[2] to ensure even distribution.
auto primary = replicas.at((size_t(id) + size_t(id) / replicas.size()) % replicas.size());
tablet_logger.debug("Primary replica of tablet {} is {}: replicas in scope: {}", id, primary, fmt::join(replicas, ","));
return primary;
}
tablet_replica tablet_map::get_primary_replica(tablet_id id, const locator::topology& topo) const {