Analysis of information sources in references of the Wikipedia article "Virtual Storage Access Method" in English language version.
Although an entry-sequenced data set does not contain an index component, alternate indexes are allowed. You can build an alternate index to keep track of these RBAs.
describes considerations for sharing VSAM data sets for NSR or LSR/GSR
Sequential (VSAM ESDS – Entry Sequenced Dataset)[dead link]
An ESDS VSAM data set contains records in the order in which they were entered
This index is called a B+ tree.