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db: snapshot: backup_task: do_backup: prioritize sstables that are already deleted from the table

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Detect SSTables that are already deleted from the table
in process_snapshot_dir when their number_of_links is equal to 1.

Note that the SSTable may be hard-linked by more than one snapshot,
so even after it is deleted from the table, its number of links
would be greater than one.  In that case, however, uploading it
earlier won't help to free-up its capacity since it is still held
by other snapshots.

Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
This commit is contained in:
Benny Halevy
2025-03-11 19:10:45 +02:00
parent 4b8699e278
commit 9a4b4afade
2 changed files with 23 additions and 0 deletions
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22
db/snapshot/backup_task.cc
View File

@@ -136,6 +136,14 @@ future<> backup_task_impl::process_snapshot_dir() {
const auto& gen = desc.generation;
_sstable_comps[gen].emplace_back(name);
++num_sstable_comps;
// When the SSTable is only linked-to by the snapshot directory,
// it is already deleted from the table's base directory, and
// therefore it better be uploaded earlier to free-up its capacity.
if (desc.component == sstables::component_type::Data && st.number_of_links == 1) {
snap_log.debug("backup_task: SSTable with generation {} is already deleted from the table", gen);
_deleted_sstables.push_back(gen);
}
} catch (const sstables::malformed_sstable_exception&) {
_files.emplace_back(name);
}
@@ -217,6 +225,20 @@ void backup_task_impl::dequeue_sstable() {
if (to_backup == _sstable_comps.end()) {
return;
}
// Prioritize stables that were already deleted
// for the table, to free up their capacity earlier.
while (!_deleted_sstables.empty()) {
auto gen = _deleted_sstables.back();
_deleted_sstables.pop_back();
auto it = _sstable_comps.find(gen);
// It is possible that the sstable was already backed up
// so silently skip this generation
// and keep looking for another candidate
if (it != _sstable_comps.end()) {
to_backup = it;
break;
}
}
auto ent = _sstable_comps.extract(to_backup);
snap_log.debug("Backing up SSTable generation {}", ent.key());
for (auto& name : ent.mapped()) {

1
db/snapshot/backup_task.hh
View File

@@ -43,6 +43,7 @@ class backup_task_impl : public tasks::task_manager::task::impl {
using comps_vector = utils::small_vector<std::string, sstables::num_component_types>;
using comps_map = std::unordered_map<sstables::generation_type, comps_vector>;
comps_map _sstable_comps;
std::vector<sstables::generation_type> _deleted_sstables;
future<> do_backup();
future<> upload_component(sstring name);