HP NonStop G-Series - Manual (Page 39)

Preparing to Use SMF
HP NonStop Storage Management Foundation User's Guide—523562-008
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Performance Impact
File Location and Placement
Most applications require no changes when the files they use become logical files.
However, applications designed to make decisions based on the assumption that the
name implies the location might not continue to work in the expected manner. In
general, such applications should execute correctly, but in some cases they might not
achieve expected performance and space distribution.
When SMF receives a file creation request, the VDP selects a volume in its associated
storage pool on which to place the file. The VDP makes the placement decision based
on available space, volume status, and configuration factors. The maximum value of
extents that can be allocated for a file created on a VDP is 953.
Process Location for Maximum Performance
To improve performance, some applications are designed to start a process in the
same processor as the primary DP2 process for a disk volume the application uses
heavily. For example, the FastSort program starts subsort processes in the processors
where its own scratch files are located to minimize interprocessor traffic. This strategy
results in optimum performance and is significant when accessing large, heavily used
files.
In an SMF environment, the volume name of the file name is also the virtual disk
process name for logically named files. The VDP’s primary process may not be in the
same processor as the physical disk where the file’s data resides. Therefore, any
application that performs as if the VDP’s primary process is in that same processor,
might locate subprocesses in the wrong processor. Even though the application
executes correctly, this choice could result in performance degradation.
Applications that select processors for execution based on file locations specified in
their name must change to ensure identical performance for direct files and logically
named files. A new criterion for the FILE_GETINFOLIST[BYNAME]_ file system
procedure encapsulates the steps needed to determine the correct processor to be
used by applications for accessing both logical and direct files. In addition, this new
criterion alleviates the need for the application to distinguish between virtual and
physical disks. For more information on the FILE_GETINFOLIST[BYNAME]_ file
system procedure, see
Section 8, Guardian Procedures .
Parallelism and Co-Location
In a non-SMF environment, files with different volume names reside on different
physical devices. In particular, all of a file’s partitions are assumed to reside on different
physical volumes because they are required to have different volume name
components. With SMF virtual disks, files with different volume names can reside on
the same physical disk if multiple virtual disks are associated with the same storage
pool, or if a mixed direct and virtual partitioning scheme is used. Therefore, multiple
partitions of a file can be on the same physical disk. Any existing application or
operations procedure that operates as if files with different volume components are on