We are using FICON Express 2Go connected on a z/800 mainframe. Does anyone know the block size of an ADRDSSU physical dump? We are wondering if we need or not software like ExHPDM (STK Product) that is able to dump on cartdrige with a 256K blocksize. Some people are saying that speed limit on an Ficon channel depending on the blocksize of the data that was carrying on.
ADRDSSU uses a 64K blocksize.
Yes, the effective speed of FICON or ESCON is affected by the blocksize. A smaller blocksize requires more CCWs and probably more I/Os to transfer the same amount of data, and there is an overhead per CCW. I believe that the overhead is worse on ESCON than FICON but it is still there on FICON. However, the max data that can be sent in a single CCW is 64K-1. To write larger blocks up to 256K you must use data chaining, which means using 4 or 5 CCWs to send the larger block. The overhead for data chaining is less than for command chaining but still not zero. So I doubt if ExHPDM will provide any significant performance gain over a direct backup, plus ExHPDM has its own overhead.
In any case, the tape drive always internally blocks user blocks (after compression) into superblocks on the tape; last number I recall for a superblock was 384K on 3590.
As someone else said, the channel is unlikely to be your bottleneck for a single backup. The disk and the tape are undoubtely slower than the channel. But if you do concurrent backups down the same disk or tape channels, you might start to stress out the channel
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