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Introduce an initial and experimental implementation of an alternative log-structured storage engine for key-value tables. Main flows and components: * The storage is composed of 32MB files, each file divided to segments of size 128k. We write to them sequentially records that contain a mutation and additional metadata. Records are written to a buffer first and then written to the active segment sequentially in 4k sized blocks. * The primary index in memory maps keys to their location on disk. It is a B-tree per-table that is ordered by tokens, similar to a memtable. * On reads we calculate the key and look it up in the primary index, then read the mutation from disk with a single disk IO. * On writes we write the record to a buffer, wait for it to be written to disk, then update the index with the new location, and free the previous record. * We track the used space in each segment. When overwriting a record, we increase the free space counter for the segment of the previous record that becomes dead. We store the segments in a histogram by usage. * The compaction process takes segments with low utilization, reads them and writes the live records to new segments, and frees the old segments. * Segments are initially "mixed" - we write to the active segment records from all tables and all tablets. The "separator" process rewrites records from mixed segments into new segments that are organized by compaction groups (tablets), and frees the mixed segments. Each write is written to the active segment and to a separator buffer of the compaction group, which is eventually flushed to a new segment in the compaction group. Currently this mode is experimental and requires an experimental flag to be enabled. Some things that are not supported yet are strong consistency, tablet migration, tablet split/merge, big mutations, tombstone gc, ttl. to use, add to config: ``` enable_logstor: true experimental_features: - logstor ``` create a table: ``` CREATE TABLE ks.t(pk int PRIMARY KEY, a int, v text) WITH storage_engine = 'logstor'; ``` INSERT, SELECT, DELETE work as expected UPDATE not supported yet no backport - new feature Closes scylladb/scylladb#28706 * github.com:scylladb/scylladb: logstor: trigger separator flush for buffers that hold old segments docs/dev: add logstor documentation logstor: recover segments into compaction groups logstor: range read logstor: change index to btree by token per table logstor: move segments to replica::compaction_group db: update dirty mem limits dynamically logstor: track memory usage logstor: logstor stats api logstor: compaction buffer pool logstor: separator: flush buffer when full logstor: hold segment until index updates logstor: truncate table logstor: enable/disable compaction per table logstor: separator buffer pool test: logstor: add separator and compaction tests logstor: segment and separator barrier logstor: separator debt controller logstor: compaction controller logstor: recovery: recover mixed segments using separator logstor: wait for pending reads in compaction logstor: separator logstor: compaction groups logstor: cache files for read logstor: recovery: initial logstor: add segment generation logstor: reserve segments for compaction logstor: index: buckets logstor: add buffer header logstor: add group_id logstor: record generation logstor: generation utility logstor: use RIPEMD-160 for index key test: add test_logstor.py api: add logstor compaction trigger endpoint replica: add logstor to db schema: add logstor cf property logstor: initial commit db: disable tablet balancing with logstor db: add logstor experimental feature flag