Millisecond response time and data set allocation

DASD can have different performance capabilities for direct access (random access, for example) and for sequential access applications. Its performance capabilities depend on whether you are reading data or writing data.

Each device type and model has a predetermined MSR capability for each condition. Additionally, if the device is attached to a cache capable control unit, the response capabilities are improved when caching is active. Therefore, each device is represented in Table 1 by eight MSR values:

Uncached Performance
- Direct Read MSR
- Direct Write MSR
- Sequential Read MSR
- Sequential Write MSR
Cache performance, DASD Fast Write performance, or both (if active)
- Direct Read MSR
- Direct Write MSR
- Sequential Read MSR
- Sequential Write MSR

If a device is cache capable, it must also have caching active at the time of allocation in order to be represented by the caching MSR values.

In table Table 1, Read is abbreviated as R and Write is abbreviated as W.

Table 1. MSR Capabilities
Device	Controller	Uncached ¹				Cached				Cached DASD Fast Write				IART²
		Direct		Sequential		Direct		Sequential		Direct		Sequential
		R	W	R	W	R	W	R	W	R	W	R	W
3350 3375	3380–1⁴	30	30	30	30	30	30	30	30	30	30	30	30	0
3380	3380–3⁴	25	25	25	25	25	25	25	25	25	25	25	25	0
	3380–13³	25	25	25	25	14	25	14	25	14	25	14	25	0
	3380–23³	25	25	25	25	10	25	10	25	10	25	10	25	0
3380 3390–1 3390–2 3390–3	3390–2⁴	25	25	25	25	25	25	25	25	25	25	25	25	0
3380 3390–1 3390–2 3390–3	3390–3/6	25	25	25	25	10	25	10	25	10	6	10	6	0
3390–9	3990–2⁴	50	50	50	50	50	50	50	50	50	50	50	50	0
3390–9	3990–3/6	50	50	50	50	35	50	25	50	35	30	25	25	0
9345	9343–x0x⁴	25	25	25	25	25	25	25	25	25	25	25	25	0
9345	9343–xCx³	10	25	10	25	10	25	10	25	10	25	10	25	0
9395	9394	10	6	10	6	10	6	10	6	10	6	10	6	0
9392–1 9392–2 9392–3	3990–3/6	10	25	10	25	10	25	10	25	10	6	10	6	0
9393 9396		10	25	10	25	10	25	10	25	10	6	10	6	0
9397		4	4	4	4	4	4	4	4	4	4	4	4	0
3995–151⁴		600	620	230	250	600	620	230	250	600	620	230	250	10
3995–153⁴		575	590	200	215	575	590	200	215	575	590	200	215	10
2105		10	6	10	6	10	6	10	6	10	6	10	6	0
2107 solid state drives		1	6	10	6	1	6	10	6	1	6	10	6	0
Note: Uncached values of a device are always used if an MSR of 999 is coded. Initial access response time (IART) values are in seconds. All others are in milliseconds. Device does not support DASD Fast Write. Device does not support either CACHE or DASD Fast Write.

From the numbers included in Table 1, you can observe that:

The fastest write performance that can be currently achieved is 6 milliseconds.
The fastest read performance that can be currently achieved is 10 milliseconds.
When cache fast write is not active, write performance is the same as for an uncached device.

You can, for example, help direct allocation to a 3390-9 by specifying an appropriate MSR. A direct MSR of 35 milliseconds with a bias of read would favor a cache-active 3390-9.

To direct allocation to a solid state drive, specify a direct MSR of 1 and a direct bias of R. Do not specify a value for sequential MSR or sequential bias. To direct storage allocation away from solid state drives, specify a direct MSR of 10 and a direct bias of R. Do not specify values for sequential MSR or sequential bias.