Specifies how many disk blocks to initially allocate to a new SAS data set
| Language element: |
data set option
|
| Default: |
enough blocks for 10 data set pages
|
| Engines: |
V6, V7, CONCUR
|
| Valid as: |
data set option, engine or host option
|
| OpenVMS specifics: |
All aspects are host-specific
|
-
allocation-quantity
-
can range from 0 to 2,147,483,647 blocks. (A block is 512 bytes.) If you
specify
ALQ=0 with the V6 or V7 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. The
default value is enough space for 10 pages.
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 may 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.
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.
