Add an optional ramcache setting to volume.config to disable it

doc/admin-guide/files/volume.config.en.rst

@@ -57,14 +57,27 @@ volumes without deleting and clearing the existing volumes.
    Changing this file to add, remove or modify volumes effectively invalidates
    the cache.
 
+
+Optional ramcache setting
+-------------------------
+
+You can also add an option ``ramcache=true/false`` to the volume configuration
+line.  True is the default setting and so not needed unless you want to explicitly
+set it.  Setting ``ramcache=false`` will disable the ramcache that normally
+sits in front of a volume.  This may be desirable if you are using something like
+ramdisks, to avoid wasting RAM and cpu time on double caching objects.
+
+
 Examples
 ========
 
 The following example partitions the cache across 5 volumes to decreasing
-single-lock pressure for a machine with few drives.::
+single-lock pressure for a machine with few drives. The last volume being
+an example of one that might be composed of purely ramdisks so that the
+ramcache has been disabled.::
 
     volume=1 scheme=http size=20%
     volume=2 scheme=http size=20%
     volume=3 scheme=http size=20%
     volume=4 scheme=http size=20%
-    volume=5 scheme=http size=20%
+    volume=5 scheme=http size=20% ramcache=false

doc/admin-guide/storage/index.en.rst

@@ -137,6 +137,15 @@ To change the RAM cache size:
    1GB or more, then restart with the :program:`trafficserver` command
    (refer to :ref:`start-traffic-server`).
 
+Disabling the RAM Cache
+-----------------------
+
+It is possible to disable the RAM cache. If you have configured your
+storage using the :file:`volume.config` you can add an optional directive
+of ``ramcache=false`` to whichever volumes you wish to have it disabled on.
+This may be desirable for volumes composed of storage like RAM disks where
+you may want to avoid double RAM caching.
+
 Changing Cache Capacity
 =======================
 

iocore/cache/Cache.cc

@@ -911,12 +911,14 @@ CacheProcessor::cacheInitialized()
         Debug("cache_init", "CacheProcessor::cacheInitialized - cache_config_ram_cache_size == AUTO_SIZE_RAM_CACHE");
         for (i = 0; i < gnvol; i++) {
           vol = gvol[i];
-          gvol[i]->ram_cache->init(vol->dirlen() * DEFAULT_RAM_CACHE_MULTIPLIER, vol);
-          ram_cache_bytes += gvol[i]->dirlen();
-          Debug("cache_init", "CacheProcessor::cacheInitialized - ram_cache_bytes = %" PRId64 " = %" PRId64 "Mb", ram_cache_bytes,
-                ram_cache_bytes / (1024 * 1024));
-          CACHE_VOL_SUM_DYN_STAT(cache_ram_cache_bytes_total_stat, (int64_t)gvol[i]->dirlen());
 
+          if (gvol[i]->cache_vol->ramcache_enabled) {
+            gvol[i]->ram_cache->init(vol->dirlen() * DEFAULT_RAM_CACHE_MULTIPLIER, vol);
+            ram_cache_bytes += gvol[i]->dirlen();
+            Debug("cache_init", "CacheProcessor::cacheInitialized - ram_cache_bytes = %" PRId64 " = %" PRId64 "Mb", ram_cache_bytes,
+                  ram_cache_bytes / (1024 * 1024));
+            CACHE_VOL_SUM_DYN_STAT(cache_ram_cache_bytes_total_stat, (int64_t)gvol[i]->dirlen());
+          }
           vol_total_cache_bytes = gvol[i]->len - gvol[i]->dirlen();
           total_cache_bytes += vol_total_cache_bytes;
           Debug("cache_init", "CacheProcessor::cacheInitialized - total_cache_bytes = %" PRId64 " = %" PRId64 "Mb",
@@ -955,14 +957,14 @@ CacheProcessor::cacheInitialized()
         for (i = 0; i < gnvol; i++) {
           vol = gvol[i];
           double factor;
-          if (gvol[i]->cache == theCache) {
+          if (gvol[i]->cache == theCache && gvol[i]->cache_vol->ramcache_enabled) {
             ink_assert(gvol[i]->cache != nullptr);
             factor = static_cast<double>(static_cast<int64_t>(gvol[i]->len >> STORE_BLOCK_SHIFT)) / theCache->cache_size;
             Debug("cache_init", "CacheProcessor::cacheInitialized - factor = %f", factor);
             gvol[i]->ram_cache->init(static_cast<int64_t>(http_ram_cache_size * factor), vol);
             ram_cache_bytes += static_cast<int64_t>(http_ram_cache_size * factor);
             CACHE_VOL_SUM_DYN_STAT(cache_ram_cache_bytes_total_stat, (int64_t)(http_ram_cache_size * factor));
-          } else {
+          } else if (gvol[i]->cache_vol->ramcache_enabled) {
             ink_release_assert(!"Unexpected non-HTTP cache volume");
           }
           Debug("cache_init", "CacheProcessor::cacheInitialized[%d] - ram_cache_bytes = %" PRId64 " = %" PRId64 "Mb", i,
@@ -2541,7 +2543,8 @@ cplist_update()
     for (config_vol = config_volumes.cp_queue.head; config_vol; config_vol = config_vol->link.next) {
       if (config_vol->number == cp->vol_number) {
         if (cp->scheme == config_vol->scheme) {
-          config_vol->cachep = cp;
+          cp->ramcache_enabled = config_vol->ramcache_enabled;
+          config_vol->cachep   = cp;
         } else {
           /* delete this volume from all the disks */
           int d_no;
@@ -2716,7 +2719,8 @@ cplist_reconfigure()
                 (int64_t)config_vol->size, 128);
         Warning("volume %d is not created", config_vol->number);
       }
-      Debug("cache_hosting", "Volume: %d Size: %" PRId64, config_vol->number, (int64_t)config_vol->size);
+      Debug("cache_hosting", "Volume: %d Size: %" PRId64 " Ramcache: %d", config_vol->number, (int64_t)config_vol->size,
+            config_vol->ramcache_enabled);
     }
     cplist_update();
     /* go through volume config and grow and create volumes */

iocore/cache/CacheHosting.cc

@@ -650,12 +650,13 @@ ConfigVolumes::BuildListFromString(char *config_file_path, char *file_buf)
     line_num++;
 
     char *end;
-    char *line_end    = nullptr;
-    const char *err   = nullptr;
-    int volume_number = 0;
-    CacheType scheme  = CACHE_NONE_TYPE;
-    int size          = 0;
-    int in_percent    = 0;
+    char *line_end        = nullptr;
+    const char *err       = nullptr;
+    int volume_number     = 0;
+    CacheType scheme      = CACHE_NONE_TYPE;
+    int size              = 0;
+    int in_percent        = 0;
+    bool ramcache_enabled = true;
 
     while (true) {
       // skip all blank spaces at beginning of line
@@ -743,6 +744,18 @@ ConfigVolumes::BuildListFromString(char *config_file_path, char *file_buf)
         } else {
           in_percent = 0;
         }
+      } else if (strcasecmp(tmp, "ramcache") == 0) { // match ramcache
+        tmp += 9;
+        if (!strcasecmp(tmp, "false")) {
+          tmp += 5;
+          ramcache_enabled = false;
+        } else if (!strcasecmp(tmp, "true")) {
+          tmp += 4;
+          ramcache_enabled = true;
+        } else {
+          err = "Unexpected end of line";
+          break;
+        }
       }
 
       // ends here
@@ -765,17 +778,19 @@ ConfigVolumes::BuildListFromString(char *config_file_path, char *file_buf)
       } else {
         configp->in_percent = false;
       }
-      configp->scheme = scheme;
-      configp->size   = size;
-      configp->cachep = nullptr;
+      configp->scheme           = scheme;
+      configp->size             = size;
+      configp->cachep           = nullptr;
+      configp->ramcache_enabled = ramcache_enabled;
       cp_queue.enqueue(configp);
       num_volumes++;
       if (scheme == CACHE_HTTP_TYPE) {
         num_http_volumes++;
       } else {
         ink_release_assert(!"Unexpected non-HTTP cache volume");
       }
-      Debug("cache_hosting", "added volume=%d, scheme=%d, size=%d percent=%d", volume_number, scheme, size, in_percent);
+      Debug("cache_hosting", "added volume=%d, scheme=%d, size=%d percent=%d, ramcache enabled=%d", volume_number, scheme, size,
+            in_percent, ramcache_enabled);
     }
 
     tmp = bufTok.iterNext(&i_state);

iocore/cache/P_CacheHosting.h

@@ -171,6 +171,7 @@ struct ConfigVol {
   CacheType scheme;
   off_t size;
   bool in_percent;
+  bool ramcache_enabled;
   int percent;
   CacheVol *cachep;
   LINK(ConfigVol, link);

iocore/cache/P_CacheVol.h

@@ -279,12 +279,13 @@ struct AIO_Callback_handler : public Continuation {
 };
 
 struct CacheVol {
-  int vol_number      = -1;
-  int scheme          = 0;
-  off_t size          = 0;
-  int num_vols        = 0;
-  Vol **vols          = nullptr;
-  DiskVol **disk_vols = nullptr;
+  int vol_number        = -1;
+  int scheme            = 0;
+  off_t size            = 0;
+  int num_vols          = 0;
+  bool ramcache_enabled = true;
+  Vol **vols            = nullptr;
+  DiskVol **disk_vols   = nullptr;
   LINK(CacheVol, link);
   // per volume stats
   RecRawStatBlock *vol_rsb = nullptr;