XL Fortran Enterprise Edition for AIX : User's Guide - IBM

Logical Volume I/O
To use a logical volume as a file, do the following:
v
v
Set up the logical volume with permissions that allow you to read or write it.
Specify the name of the special file (for example, /dev/rlv99) in the OPEN
statement.
Attention: Do not perform this kind of I/O with any logical volume that already
contains a file system; doing so will destroy the file system. You must also take
any precautions necessary to ensure that multiple users or programs do not write
to the same logical volume or write to a logical volume while someone else is
reading from it.
Notes:
1. A logical volume can only be opened as a single direct-access file with a record
length that is a multiple of the logical volume’s sector size (usually 512 bytes).
2. I/O operations are not guaranteed to detect attempts to read or write past the
end of the logical volume. Therefore, make sure that the program keeps track
of the extent of the logical volume. The maximum amount of data that can be
stored this way on logical volume is the size of the logical volume minus the
size of one stripe. The XL Fortran I/O routines use this stripe for bookkeeping.
3. For optimal performance of data striping, ensure that any data items that you
specified in the read or write lists for a logical volume are aligned on 64-byte
boundaries. The simplest way to ensure this alignment for large static arrays
and common blocks is to specify the option -qalign=4k.
4. Regardless of any STATUS=’SCRATCH’ or STATUS=’DELETE’ specifiers,
neither the data in a logical volume nor the special file in /dev is destroyed by
an OPEN or CLOSE statement.
Related Information: See “-qalign Option” on page 125.
Data Striping
Data striping is primarily useful for increasing I/O throughput for large,
direct-access scratch files. The performance benefit is greatest when a program
reads and writes large objects.
When you make use of data striping, you perform I/O to a logical volume as
described in “Logical Volume I/O” and set up the logical volume especially for
high-performance striped I/O through the smit or mklv commands. You can then
use the technique that is described in “Naming Scratch Files” on page 332 to place
a scratch file on a striped logical volume.
For example, consider a Fortran program that contains the following statements:
OPEN (UNIT=42, STATUS=’SCRATCH’,
+ FORM=’UNFORMATTED’, ACCESS=’DIRECT’, RECL=131072)
...
OPEN (UNIT=101, STATUS=’SCRATCH’,
+ FORM=’UNFORMATTED’, ACCESS=’DIRECT’, RECL=131072)
You could place the scratch files for units 42 and 101 on the raw logical volumes
/dev/rlv30 and /dev/rlv31 by setting environment variables before running the
program, as follows:
XLFRTEOPTS="scratch_vars=yes"
XLFSCRATCH_42="/dev/rlv30"
XLFSCRATCH_101="/dev/rlv31"
export XLFRTEOPTS XLFSCRATCH_42 XLFSCRATCH_101
334 XL Fortran Enterprise Edition for AIX : User’s Guide