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IMBED causes VSAM to place the sequence set in the first track of each CA and to replicate the sequence set around the track until it is full. This option normally allows one seek to satisfy the read for both the sequence set and the data it references, but it increases the amount of space required to contain the file by a minimum of about seven percent and reduces the file integrity. It can be up to 50 percent, if the subcylinder allocations are used. The latter occurs because VSAM must now update the indexes multiple times.
A typical problem with IMBED is records that are sometimes not found. This occurs because not all of the sequence sets on the track are updated. This only happens when the file is updated from two regions at once without the use of SHAREOPTIONS=4; but this is not uncommon, as few shops can live with the poor performance of SHAREOPTIONS=4. The use of IMBED is not recommended. If performance dictates, place the index and data portions of the cluster on separate packs. VPE warns you if IMBED is specified for a file that is larger than 200 cylinders.
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