--- a/dom/media/MediaCache.cpp
+++ b/dom/media/MediaCache.cpp
@@ -14,16 +14,17 @@
 #include "mozilla/Logging.h"
 #include "mozilla/Preferences.h"
 #include "FileBlockCache.h"
 #include "nsIObserverService.h"
 #include "nsISeekableStream.h"
 #include "nsIPrincipal.h"
 #include "mozilla/Attributes.h"
 #include "mozilla/Services.h"
+#include "mozilla/Telemetry.h"
 #include <algorithm>
 
 namespace mozilla {
 
 #undef LOG
 #undef LOGI
 LazyLogModule gMediaCacheLog("MediaCache");
 #define LOG(...) MOZ_LOG(gMediaCacheLog, LogLevel::Debug, (__VA_ARGS__))
@@ -121,16 +122,21 @@ public:
   ~MediaCache() {
     NS_ASSERTION(mStreams.IsEmpty(), "Stream(s) still open!");
     Truncate();
     NS_ASSERTION(mIndex.Length() == 0, "Blocks leaked?");
     if (mFileCache) {
       mFileCache->Close();
       mFileCache = nullptr;
     }
+    LOG("MediaCache::~MediaCache(this=%p) MEDIACACHE_WATERMARK_KB=%u",
+        this, unsigned(mIndexWatermark * MediaCache::BLOCK_SIZE / 1024));
+    Telemetry::Accumulate(
+      Telemetry::HistogramID::MEDIACACHE_WATERMARK_KB,
+      uint32_t(mIndexWatermark * MediaCache::BLOCK_SIZE / 1024));
     MOZ_COUNT_DTOR(MediaCache);
   }
 
   // Main thread only. Creates the backing cache file. If this fails,
   // then the cache is still in a semi-valid state; mFD will be null,
   // so all I/O on the cache file will fail.
   nsresult Init();
   // Shut down the global cache if it's no longer needed. We shut down
@@ -343,16 +349,18 @@ protected:
   // readers that need to block will Wait() on this monitor. When new
   // data becomes available in the cache, we NotifyAll() on this monitor.
   ReentrantMonitor         mReentrantMonitor;
   // This is only written while on the main thread and the monitor is held.
   // Thus, it can be safely read from the main thread or while holding the monitor.
   nsTArray<MediaCacheStream*> mStreams;
   // The Blocks describing the cache entries.
   nsTArray<Block> mIndex;
+  // Keep track for highest number of blocks used, for telemetry purposes.
+  int32_t mIndexWatermark = 0;
   // Writer which performs IO, asynchronously writing cache blocks.
   RefPtr<FileBlockCache> mFileCache;
   // The list of free blocks; they are not ordered.
   BlockList       mFreeBlocks;
   // True if an event to run Update() has been queued but not processed
   bool            mUpdateQueued;
 #ifdef DEBUG
   bool            mInUpdate;
@@ -715,16 +723,17 @@ MediaCache::FindBlockForIncomingData(Tim
     // b) the data we're going to store in the free block is not higher
     // priority than the data already stored in the free block.
     // The latter can lead us to go over the cache limit a bit.
     if ((mIndex.Length() < uint32_t(GetMaxBlocks()) || blockIndex < 0 ||
          PredictNextUseForIncomingData(aStream) >= PredictNextUse(aNow, blockIndex))) {
       blockIndex = mIndex.Length();
       if (!mIndex.AppendElement())
         return -1;
+      mIndexWatermark = std::max(mIndexWatermark, blockIndex + 1);
       mFreeBlocks.AddFirstBlock(blockIndex);
       return blockIndex;
     }
   }
 
   return blockIndex;
 }
 

--- a/toolkit/components/telemetry/Histograms.json
+++ b/toolkit/components/telemetry/Histograms.json
@@ -8325,16 +8325,26 @@
     "record_in_processes": ["main", "content"],
     "alert_emails": ["cpearce@mozilla.com"],
     "bug_numbers": [1366639],
     "expires_in_version": "60",
     "kind": "enumerated",
     "n_values": 50,
     "description": "Counts of the maximum number of shared memory buffers used for transferring video frames between the CDM and Gecko processes during playback of DRM'd video. Reported once per CDMVideoDecoder instance, i.e., once per JavaScript SourceBuffer during playback of video using EME."
   },
+  "MEDIACACHE_WATERMARK_KB": {
+    "record_in_processes": ["main", "content"],
+    "alert_emails": ["gsquelart@mozilla.com"],
+    "bug_numbers": [1366929],
+    "expires_in_version": "60",
+    "kind": "linear",
+    "high": 520000,
+    "n_buckets": 66,
+    "description": "Maximum MediaCache buffer size reached, in KB. Recorded at every MediaCache destruction, i.e., whenever there is no more media data to be downloaded or kept for playback."
+  },
   "VIDEO_MFT_OUTPUT_NULL_SAMPLES": {
     "record_in_processes": ["main", "content"],
     "alert_emails": ["cpearce@mozilla.com"],
     "expires_in_version": "53",
     "kind": "enumerated",
     "n_values": 10,
     "description": "Does the WMF video decoder return success but null output? 0 = playback successful, 1 = excessive null output but able to decode some frames, 2 = excessive null output and gave up, 3 = null output but recovered, 4 = non-excessive null output without being able to decode frames.",
     "bug_numbers": [1176071]