9.2.1. SSD file systems

In the UNICOS operating system, file storage space is divided into file systems. A file system is a logical device made up of slices from various physical devices. A slice is a set of consecutive cylinders or blocks. Each file system is mounted on a directory name so that users can access the file system through the directory name. Thus, if a file system is composed of SSD slices, any file or its descendants that are written into the associated directory will reside on SSD.

To use an SSD file system from a Fortran program, users must ensure that the path name of the file contains the appropriate directory. For example, if an SSD resident file system is mounted on the /tmp directory, use the assign(1) command to assign a file to that directory and the file will reside on the SSD.

Example:

assign -a /tmp/ssdfile u:10

Users can also use the OPEN statement in the program to open a file in the directory.

SSD file systems are useful for holding frequently referenced files such as system binary files and object libraries. Some sites use an SSD file system for system swapping space such as /drop or /swapdev. Finally, SSD file systems can be used as a fast temporary scratch space.

9.2.2. Secondary data segments (SDS)

The secondary data segment (SDS) feature allows the I/O routines to treat part of the SSD like an extended or secondary memory. SDS allows I/O requests to move directly between memory and SSD; this provides sustained transfer rates that are faster than that of SSD file systems.

Users must explicitly request SDS space for a process but the space is released automatically when the program ends. Users can request that several files reside in SDS space but the total amount of SDS space requested for the files must be within the SDS allocation limit for the user.

To request SDS space for unit 11 from a Fortran program, use either of the following assign commands:

assign -F cos,sds u:11

assign -F cachea.sds u:11

The ssread(2) and sswrite system calls can be called from a Fortran program to move data between a buffer and SDS directly. ssread, sswrite, and ssbreak should not be used in a Fortran program that accesses SDS through the assign command because the libraries use SDSALLOC(3F) to control SDS allocation. Using SSBREAK directly from Fortran conflicts with the SDS management provided by SDSALLOC. The UNICOS System Calls Reference Manual, describes ssbreak, ssread, and sswrite.

On UNICOS/mk systems, the library does not handle allocation of SDS space from more than one processing element (PE). For files opened from different PEs, do not use SDSALLOC, assign -F sds, or the sds option of assign -F cache or assign -F cachea.

A Fortran programmer can use the CDIR$ AUXILIARY compiler directive to assign SDS space to the arrays specified on the directive line. The name of an auxiliary array or variable must not appear in an I/O statement. See the Fortran Language Reference manuals for your compiler system for a description of this feature. The UNICOS File Formats and Special Files Reference Manual, describes SDS.

9.2.3. Logical device cache (ldcache)

The system administrator can allocate a part of the SDS space as ldcache. ldcache is a buffer space for the most heavily-used disk file systems. It is assigned one file system at a time. Allocation of the units within each assigned space is done on a least recently used basis. When a given file system's portion of the ldcache is full, the least recently accessed units are flushed to disk. You do not need to change a Fortran program to make use of ldcache. The program or operating system issues physical I/O requests to disk.