Drop some notes regarding required refactoring, add grown indicator

Ref #10

src/fellow_cache.c

@@ -588,7 +588,6 @@ struct fellow_busy {
 	size_t				sz_returned;
 	// we add more whenever we exceed estimate
 	size_t				sz_increment;
-
 	// how much we already alloced on dsk
 	size_t				sz_dskalloc;
 
@@ -601,7 +600,7 @@ struct fellow_busy {
 #define FCO_MAX_REGIONS ((FELLOW_DISK_LOG_BLOCK_ENTRIES - 1) * DLE_REG_NREGION)
 	struct buddy_off_extent	region[FCO_MAX_REGIONS];
 	unsigned			nregion;
-
+	uint8_t				grown;
 #define FBO_NIO 11
 	/* protected by fco mtx */
 	uint8_t				io_idx;
@@ -3569,6 +3568,8 @@ fellow_busy_obj_getspace(struct fellow_busy *fbo, size_t *sz, uint8_t **ptr)
 
 		fbo->sz_estimate += fbo->sz_increment;
 		fbo->sz_increment <<= 1;
+		// XXX wip
+		fbo->grown = 1;
 	}
 
 	fcs = fbo->body_seg;
@@ -3946,15 +3947,34 @@ fellow_busy_free(struct fellow_busy **fbop)
  *
  * the trouble with disk vs. mem segments
  *
- * the log assumes we got max 51 disk segments per obj
+ * the log assumes we got max FCO_MAX_REGIONS = 220 regions per obj
  * our memory segments must not be larger than 1/16th - 1/8 of mem
  *
  * so: alloc one big chunk of disk, chop into smaller, individually
  * checksummed segments
  *
+ * XXX CHANGE:
  * for chunked: next pow2 with every chunk, so we never get above ~50
  * disk segments
  *
+ * Ballpark
+ * - disk/mem = 10k
+ * min:
+ * - mem =  1GB -> max seg 128MB
+ * - dsk = 10TB -> max obj 1.28TB
+ * max:
+ * - max seg because I/O 1<<30 = 1 GB
+ * - dsk = 16EB -> max obj 1<<61 2EB
+ * - region sz -> 2EB / 220 = 9PB (16PB/8PB)
+ * - seg/obj -> 1<<31 = 2g
+ * - sizeof(fds) = 56
+ * - size of all segments in seglists: 112GB
+ *
+ * TODO:
+ * - region != segment
+ * - fbo.io independent of FCO_MAX_REGIONS
+ * - introduce max object size depending on dsk size
+ * - change region allocation such that max object will always fit
  */
 
 // XXX rearrange seglists ?