add RewindBuf.FullnessRatio to let you know how full it is. that, and RewindBuf.Count will give you useful metrics to gauge your choice of rewind buffer sizes against desired rewind timespan

BizHawk.MultiClient/Rewind.cs

@@ -33,7 +33,8 @@ namespace BizHawk.MultiClient
 					
 					//I checked the DeleteOnClose operation to make sure it cleans up when the process is aborted, and it seems to.
 					//Otherwise we would have a more complex tempfile management problem here.
-					mStream = new FileStream(path, FileMode.Create, System.Security.AccessControl.FileSystemRights.FullControl, FileShare.None, 512, FileOptions.DeleteOnClose);
+					//4KB buffer chosen due to similarity to .net defaults, and fear of anything larger making hiccups for small systems (we could try asyncing this stuff though...)
+					mStream = new FileStream(path, FileMode.Create, System.Security.AccessControl.FileSystemRights.FullControl, FileShare.None, 4*1024, FileOptions.DeleteOnClose);
 				}
 				else
 				{
@@ -54,14 +55,32 @@ namespace BizHawk.MultiClient
 			Stream mStream;
 			LinkedList<ListItem> mBookmarks = new LinkedList<ListItem>();
 			LinkedListNode<ListItem> mHead, mTail;
-			long mCapacity;
+			long mCapacity, mSize;
 
+			/// <summary>
+			/// Returns the amount of the buffer that's used
+			/// </summary>
+			public long Size { get { return mSize; } }
+
+			/// <summary>
+			/// Gets the current fullness ratio (Size/Capacity). Note that this wont reach 100% due to the buffer size not being a multiple of a fixed savestate size.
+			/// </summary>
+			public float FullnessRatio { get { return (float)((double)Size / (double)mCapacity); } }
+
+			/// <summary>
+			/// the number of frames stored here
+			/// </summary>
 			public int Count { get { return mBookmarks.Count; } }
+
+			/// <summary>
+			/// The underlying stream to 
+			/// </summary>
 			public Stream Stream { get { return mStream; } }
 
 			public void Clear()
 			{
 				mHead = mTail = null;
+				mSize = 0;
 				mBookmarks.Clear();
 			}
 
@@ -88,6 +107,8 @@ namespace BizHawk.MultiClient
 
 			public long Enqueue(int timestamp, int amount)
 			{
+				mSize += amount;
+
 				if (mHead == null)
 				{
 					mTail = mHead = mBookmarks.AddFirst(new ListItem(timestamp, 0, amount));
@@ -130,6 +151,7 @@ namespace BizHawk.MultiClient
 					if (mHead.Value.endExclusive > mTail.Value.index && mHead.Value.index <= mTail.Value.index)
 					{
 						LinkedListNode<ListItem> nextTail = mTail.Next;
+						mSize -= mTail.Value.length;
 						mBookmarks.Remove(mTail);
 						mTail = nextTail;
 					}
@@ -143,6 +165,7 @@ namespace BizHawk.MultiClient
 			{
 				if (mHead == null) throw new InvalidOperationException("Attempted to pop from an empty data structure");
 				var ret = mHead.Value;
+				mSize -= ret.length;
 				LinkedListNode<ListItem> nextHead = mHead.Previous;
 				mBookmarks.Remove(mHead);
 				if (mHead == mTail)
@@ -157,6 +180,7 @@ namespace BizHawk.MultiClient
 			{
 				if (mTail == null) throw new InvalidOperationException("Attempted to dequeue from an empty data structure");
 				var ret = mTail.Value;
+				mSize -= ret.length;
 				LinkedListNode<ListItem> nextTail = mTail.Next;
 				mBookmarks.Remove(mTail);
 				if (mTail == mHead)