Determining the storage cost of enabling HPDT Packing

HPDT Packing is enabled on a TRLE basis. When enabled, seven packing buffers are acquired for each write device defined in the TRLE. Packing buffer size does not vary within the TRLE. Packing buffers are obtained from the smallest CSM data space pool that can contain the entire buffer.

Determining packing buffer size

The packing buffer size is equal to the number of 4 KB pages (minus 1 page) specified by the MAXBFRU parameter on the TRLE definition in the adjacent link station (or equivalent when the adjacent link station is a router). This value can be determined without accessing the adjacent link station when the TRLE is active. Issue the DISPLAY NET,ID=trlename command for the TRLE for which you are considering enabling HPDT Packing. The MAXBFRU value of the adjacent link station is contained in the response. The response looks similar to the following example:

IST097I DISPLAY ACCEPTED                                      
 IST075I NAME = TRLE1A, TYPE = TRLE                            
 IST486I STATUS= ACTIV----E, DESIRED STATE= ACTIV              
 IST087I TYPE = LEASED             , CONTROL = MPC , HPDT = YES
 IST1715I MPCLEVEL = HPDT       MPCUSAGE = SHARE               
 IST1717I ULPID = AHHCPU1                                       
 IST1801I UNITS OF WORK FOR NCB AT ADDRESS X'02EAE018'          
 IST1802I CURRENT = 0 AVERAGE = 1 MAXIMUM = 2                   
>IST1577I HEADER SIZE = 4092 DATA SIZE = 16 STORAGE = ***NA***  
 IST1221I WRITE DEV = 0508 STATUS = ACTIVE     STATE = ONLINE   
 IST1577I HEADER SIZE = 4092 DATA SIZE = 16 STORAGE = DATASPACE 
 IST1221I READ  DEV = 0408 STATUS = ACTIVE     STATE = ONLINE   
 IST314I END

The DATA SIZE value reported in the first IST1577I message reflects the MAXBFRU parameter in the adjacent link station. In this case, the adjacent link station defined a data stream maximum size of 16 pages. Therefore, if HPDT Packing is enabled for this TRLE, the packing buffer size will be 15 pages, or 60 KB. Because 7 packing buffers are allocated for the single write device, the total storage cost in CSM fixed data space is 7*60 KB or 420 KB.

Packing buffer size versus CSM data space pool size

The packing buffers are obtained from the smallest CSM data space pool that can contain the entire buffer. If the packing buffer size does not equal one of the CSM data space pool sizes, storage is wasted.

In the example above, the packing buffer size is exactly equal to the second largest CSM data space pool size; therefore no storage is wasted. The worst case occurs when DATA SIZE is equal to 10, meaning 9 page (36 KB) packing buffers are used. Because the packing buffers cannot be acquired from the 32 KB pool, they are acquired from the 60 KB pool. The last 24 KB of each packing buffer is wasted. 7*24 KB equals 160 KB of wasted storage for a single write device.

The previous example explains why you should adjust the MAXBFRU value in the adjacent node (if necessary) before enabling HPDT Packing, so that the packing buffer size is equal to one of the CSM data space pools sizes.

Table 1 shows the correlation between DATA SIZE (MAXBFRU in the adjacent link station), the packing buffer size, the CSM data space pool size from which the packing buffers are allocated, and any apparent storage waste because of mismatch of packing buffer size and CSM data space pool size.

Table 1. HPDT Packing - packing buffer size, CSM pools size, and waste per packing buffer
DATA SIZE	Packing buffer size	CSM pool used	Waste per packing buffer
2	4 KB	4 KB	None
3	8 KB	16 KB	8 KB
4	12 KB	16 KB	4 KB
5	16 KB	16 KB	None
6	20 KB	32 KB	12 KB
7	24 KB	32 KB	8 KB
8	28 KB	32 KB	4 KB
9	32 KB	32 KB	None
10	36 KB	60 KB	24 KB
11	40 KB	60 KB	20 KB
12	44 KB	60 KB	16 KB
13	48 KB	60 KB	12 KB
14	52 KB	60 KB	8 KB
15	56 KB	60 KB	4 KB
16	60 KB	60 KB	None