VSAM (Virtual Storage Access Method) and QSAM (Queued Sequential Access Method) are both access methods on IBM mainframes, but they serve different purposes. VSAM supports key-based and position-based access and multiple dataset types (KSDS, ESDS, RRDS, LDS). QSAM is for sequential datasets only: you read or write records in order, one after another. This page compares VSAM and QSAM so you can choose the right one for your workload.
QSAM (Queued Sequential Access Method) is used for sequential datasets. The program opens the file and then reads the next record or writes the next record. Records are processed in physical order. The dataset has a record format (RECFM) and block size (BLKSIZE); the system blocks and deblocks records. QSAM datasets can be on DASD or tape. They can be created with JCL allocation (DISP=NEW, SPACE=...) and can be browsed and edited with ISPF 3.4. There is no index and no way to "read record with key X"; you read next, period.
VSAM organizes data in control intervals and (for KSDS) maintains an index. You can read sequentially (in key order for KSDS or physical order for ESDS) or randomly by key (KSDS), RBA (ESDS), or RRN (RRDS). VSAM datasets are created with IDCAMS, exist only on DASD, and use a proprietary format so standard ISPF edit/browse doesn’t show records in a friendly way. VSAM supports sharing (e.g. CICS and batch) via SHAREOPTIONS.
| Aspect | VSAM | QSAM |
|---|---|---|
| Access | Sequential and random (key, RBA, RRN) | Sequential only |
| Structure | Control intervals, index (KSDS), catalog | Blocks, RECFM, no index |
| Creation | IDCAMS DEFINE CLUSTER | JCL allocation or program |
| Storage | DASD only | DASD or tape |
| ISPF browse/edit | No (proprietary format) | Yes |
Use QSAM when the access pattern is strictly sequential: read the whole file from start to end, or write the whole file from start to end. Examples: reading a tape input file, writing a report to SYSOUT or a dataset, producing a flat file for another system, writing a log. Use QSAM when the dataset must be on tape or when you want to browse or edit the data with ISPF. Use QSAM when you don’t need key-based lookups or in-place updates; the simplicity of "next record" is enough.
Use VSAM when you need to find a record by key (e.g. customer ID) and read, update, or delete it. Use VSAM when you need RBA-based access (ESDS) or slot-based access (RRDS). Use VSAM when the same file is used by both batch and online and you want one copy with SHAREOPTIONS. Use VSAM when you have a mix of sequential passes and random lookups. VSAM is the wrong choice if you only need one-way sequential and want tape or ISPF edit.
Yes. A VSAM KSDS can be read sequentially in key order; the program issues "read next" and gets records in ascending key order. An ESDS can be read sequentially in the order records were written. So "sequential access" is not unique to QSAM; the difference is that VSAM also supports random access. If your program only ever does sequential read (e.g. a batch report that walks the whole file in key order), you could use either a VSAM KSDS or a sequential file; the choice then depends on whether you need the file on tape, ISPF browse, or key access elsewhere.
QSAM is like reading a book from page 1 to the end: you go in order and you can't jump to "the page about dogs" unless you read until you find it. VSAM is like a book with an index at the back: you can look up "dogs" and go straight to that page, and you can also read from front to back if you want. So QSAM = only in order; VSAM = in order or jump to a key or position.
1. Which access method is sequential-only (no key or RBA lookup)?
2. You need to find a record by customer ID. Which is appropriate?