Data Set Options under OpenVMS

ALQ= Data Set Option: OpenVMS

Specifies how many disk blocks to initially allocate to a new SAS data set.

Default:	enough blocks for 10 data set pages
Valid in:	DATA step and PROC steps
Category:	Data Set Control
Engines:	V9, V8, V7, CONCUR
OpenVMS specifics:	All aspects are host-specific

Syntax
Details
See Also

Syntax

ALQ=allocation-quantity

allocation-quantity

specifies the number of blocks to allocate. The default value is enough space for ten pages. If you specify ALQ=0 with the V6 or V8 engine, OpenVMS uses the minimum number of blocks that are required for a sequential file. If you specify ALQ=0 with the CONCUR engine, OpenVMS uses the minimum number of blocks that are required for a relative file. OpenVMS RMS always rounds the ALQ= value up to the next disk cluster boundary.

Range:

0 - 2,147,483,647 blocks (A block is 512 bytes.)

Details

The ALQ= data set option is often used with the BUFSIZE= and DEQ= data set options to control buffer size and disk allocation for more efficient I/O. For example, if an application processes a large SAS data set sequentially, it might be more efficient to use option values like the following:

data mylib.a(bufsize=16384 alq=300 deq=96);
   . . . more data lines . . .
run;

These statements tell SAS to use a buffer size of 16,384 bytes for data set I/O and to allocate 300 disk blocks to the file initially; when more space is needed, it is allocated in 96-block chunks. This potentially reduces the number of file extensions required for the file. Unused disk blocks are deallocated and freed to the file system at close time. This reduces the number of times a file must be extended, potentially creates a more contiguous file, and reduces subsequent access time.

The order of precedence (from highest to lowest) for specifying initial-allocation values and file-extension values is

an ALQ= or DEQ= data set option specified for a specific data set
an ALQ= or DEQ= data set option specified in a LIBNAME statement or function
system default values.

The ALQ= data set option also corresponds to the FAB$L_ALQ field in the OpenVMS RMS structure. For information about the FAB$L_ALQ field, see Guide to OpenVMS File Applications.