db: allow configuring a column family to be memory-only

Useful for tests.

database.cc

@@ -527,6 +527,8 @@ column_family::config
 keyspace::make_column_family_config(const schema& s) const {
     column_family::config cfg;
     cfg.datadir = column_family_directory(s.cf_name(), s.id());
+    cfg.enable_disk_reads = _config.enable_disk_reads;
+    cfg.enable_disk_writes = _config.enable_disk_writes;
     return cfg;
 }
 

database.hh

@@ -79,6 +79,8 @@ class column_family {
 public:
     struct config {
         sstring datadir;
+        bool enable_disk_writes = true;
+        bool enable_disk_reads = true;
     };
 private:
     schema_ptr _schema;
@@ -150,6 +152,8 @@ class keyspace {
 public:
     struct config {
         sstring datadir;
+        bool enable_disk_reads = true;
+        bool enable_disk_writes = true;
     };
 private:
     std::unique_ptr<locator::abstract_replication_strategy> _replication_strategy;

tests/urchin/mutation_test.cc

@@ -209,6 +209,8 @@ SEASTAR_TEST_CASE(test_multiple_memtables_one_partition) {
         {{"p1", utf8_type}}, {{"c1", int32_type}}, {{"r1", int32_type}}, {}, utf8_type));
 
     column_family::config cfg;
+    cfg.enable_disk_reads = false;
+    cfg.enable_disk_writes = false;
     column_family cf(s, cfg);
 
     const column_definition& r1_col = *s->get_column_definition("r1");
@@ -246,6 +248,8 @@ SEASTAR_TEST_CASE(test_multiple_memtables_multiple_partitions) {
         {{"p1", int32_type}}, {{"c1", int32_type}}, {{"r1", int32_type}}, {}, utf8_type));
 
     column_family::config cfg;
+    cfg.enable_disk_reads = false;
+    cfg.enable_disk_writes = false;
     column_family cf(s, cfg);
     std::map<int32_t, std::map<int32_t, int32_t>> shadow, result;