IBM 3350 Direct Access Storage

Geometry and Capacity

The 3350 provides 555 data cylinders with 30 data tracks per cylinder, for 16,650 normally usable data tracks. Additional alternate capacity is reserved for defect handling rather than normal dataset allocation.

30data tracks per cylinder

555data cylinders per drive

16,650data tracks per drive

19,254track-capacity calculation value

19,069maximum single unkeyed block

572,070maximum data bytes per cylinder

317,498,850maximum data bytes per drive

19,254 versus 19,069: 19,254 is the value used in the 3350 records-per-track capacity calculation. A single unkeyed record incurs 185 bytes of record overhead, leaving a maximum data length of 19,069 bytes: 19,254 - 185 = 19,069.

The published 555-cylinder capacity excludes alternate cylinders. Alternate space and the 3350's defect-skipping facilities are part of the device's defect-handling scheme and should not be treated as ordinary allocatable data space.

Records per Track

For equal-length records, the number of records that fit on a 3350 track is determined from the track-capacity value and the key, data, and per-record overhead:

                    19,254
Records per Track = ----------------
                    KL + DL + C

KL = key length
DL = data length

C  = 185 when KL = 0
C  = 267 when KL > 0

The result is truncated to a whole number of records. For an unkeyed record, the maximum data length for a specified records-per-track value is therefore:

DLmax = INT(19,254 / records-per-track) - 185

For a keyed record, the corresponding table value represents the combined key plus data length:

(KL + DL)max = INT(19,254 / records-per-track) - 267

The 19,254-byte calculation value includes the device-capacity allowances associated with track formatting, including Home Address, Record Zero, and defect skipping. It should not be interpreted as the maximum length of a single user data block.

3350 Capacity Table

The table gives the range of record sizes producing each records-per-track value. For unkeyed records the range is the data length (DL). For keyed records it is the combined key and data length (KL + DL). The final two columns show the resulting number of records for a full 30-track cylinder and a 555-cylinder drive.

Records per Track	Without Keys - DL		With Keys - KL + DL		Records per Cylinder	Records per Drive
Records per Track	Min	Max	Min	Max	Records per Cylinder	Records per Drive
1	9,443	19,069	9,361	18,987	30	16,650
2	6,234	9,442	6,152	9,360	60	33,300
3	4,629	6,233	4,547	6,151	90	49,950
4	3,666	4,628	3,584	4,546	120	66,600
5	3,025	3,665	2,943	3,583	150	83,250
6	2,566	3,024	2,484	2,942	180	99,900
7	2,222	2,565	2,140	2,483	210	116,550
8	1,955	2,221	1,873	2,139	240	133,200
9	1,741	1,954	1,659	1,872	270	149,850
10	1,566	1,740	1,484	1,658	300	166,500
11	1,420	1,565	1,338	1,483	330	183,150
12	1,297	1,419	1,215	1,337	360	199,800
13	1,191	1,296	1,109	1,214	390	216,450
14	1,099	1,190	1,017	1,108	420	233,100
15	1,019	1,098	937	1,016	450	249,750
16	948	1,018	866	936	480	266,400
17	885	947	803	865	510	283,050
18	829	884	747	802	540	299,700
19	778	828	696	746	570	316,350
20	732	777	650	695	600	333,000
21	691	731	609	649	630	349,650
22	653	690	571	608	660	366,300
23	618	652	536	570	690	382,950
24	586	617	504	535	720	399,600
25	556	585	474	503	750	416,250
26	529	555	447	473	780	432,900
27	503	528	421	446	810	449,550
28	479	502	397	420	840	466,200
29	457	478	375	396	870	482,850
30	437	456	355	374	900	499,500
31	417	436	335	354	930	516,150
32	399	416	317	334	960	532,800
33	382	398	300	316	990	549,450
34	366	381	284	299	1,020	566,100
35	350	365	268	283	1,050	582,750
36	336	349	254	267	1,080	599,400
37	322	335	240	253	1,110	616,050
38	309	321	227	239	1,140	632,700
39	297	308	215	226	1,170	649,350
40	285	296	203	214	1,200	666,000
41	274	284	192	202	1,230	682,650
42	263	273	181	191	1,260	699,300
43	253	262	171	180	1,290	715,950
44	243	252	161	170	1,320	732,600
45	234	242	152	160	1,350	749,250
46	225	233	143	151	1,380	765,900
47	217	224	135	142	1,410	782,550
48	208	216	126	134	1,440	799,200
49	201	207	119	125	1,470	815,850
50	193	200	111	118	1,500	832,500
51	186	192	104	110	1,530	849,150
52	179	185	97	103	1,560	865,800
53	172	178	90	96	1,590	882,450
54	166	171	84	89	1,620	899,100
55	159	165	77	83	1,650	915,750
56	153	158	71	76	1,680	932,400
57	147	152	65	70	1,710	949,050
58	142	146	60	64	1,740	965,700
59	136	141	54	59	1,770	982,350
60	131	135	49	53	1,800	999,000
61	126	130	44	48	1,830	1,015,650
62	121	125	39	43	1,860	1,032,300
63	116	120	34	38	1,890	1,048,950
64	112	115	30	33	1,920	1,065,600
65	107	111	25	29	1,950	1,082,250
66	103	106	21	24	1,980	1,098,900
67	99	102	17	20	2,010	1,115,550
68	95	98	13	16	2,040	1,132,200
69	91	94	9	12	2,070	1,148,850
70	87	90	5	8	2,100	1,165,500
71	83	86		4	2,130	1,182,150
72	79	82			2,160	1,198,800
73	76	78			2,190	1,215,450
74	72	75			2,220	1,232,100
75	69	71			2,250	1,248,750
76	66	68			2,280	1,265,400
77	62	65			2,310	1,282,050
78	59	61			2,340	1,298,700
79	56	58			2,370	1,315,350
80	53	55			2,400	1,332,000
81	50	52			2,430	1,348,650
82	47	49			2,460	1,365,300
83	45	46			2,490	1,381,950
84	42	44			2,520	1,398,600
85	39	41			2,550	1,415,250
86	37	38			2,580	1,431,900
87	34	36			2,610	1,448,550
88	32	33			2,640	1,465,200
89	29	31			2,670	1,481,850
90	27	28			2,700	1,498,500
91	25	26			2,730	1,515,150
92	23	24			2,760	1,531,800
93	20	22			2,790	1,548,450
94	18	19			2,820	1,565,100
95	16	17			2,850	1,581,750
96	14	15			2,880	1,598,400
97	12	13			2,910	1,615,050
98	10	11			2,940	1,631,700
99	8	9			2,970	1,648,350
100	6	7			3,000	1,665,000
101	4	5			3,030	1,681,650
102	2	3			3,060	1,698,300
103	1	1			3,090	1,714,950

Blank keyed entries indicate that the 267-byte keyed-record overhead leaves no positive key-plus-data length for that records-per-track value.