DCOM can move open files for all releases above D45, otherwise, it does not move open
files.
◦
DCOM burdens disk I/O channels and can degrade the performance of important disk
I/O applications. Running DCOM at a low priority reduces (but does not eliminate)
performance degradation.
◦
DCOM rearranges file extents. Any file that was specially allocated in contiguous extents
to reduce disk seek time might end up in a noncontiguous state after it is compressed.
•
DCOM is designed for online disks so it does not perform perfect compression. You can
sometimes achieve additional compression by running DCOM several times (serially), but you
need to consider the performance implications. The starting overhead of each run is significant
because DCOM must read and sort the disk directory. DCOM cannot determine when
additional runs are pointless.
•
DCOM prevents more than one DCOM process from running at a time by exclusively opening
the SYSDCOM.RECOVERY file to signify that it is running. This error message occurs if a
second DCOM process is attempted:
Unable to open SYSDCOM.RECOVERY file, error: 12
•
DCOM allocates space for data segment and swap file for storing data about file labels and
extents. When DCOM needs to allocate 1024 MB of workspace, it displays "
WARNING:
1024MB SPACE WILL BE REQUIRED
". If the required space is not available, DCOM
abends and displays "
DIRECTORY WORKFILE CREATION/OPEN/MAPPING ERROR: 43.
TRY AGAIN WITH 1024MB WORKSPACE
".
•
If the system crashes while DCOM is running, temporary files could be left on the disk being
compressed. The files are then owned by the super ID (255,255) and secured "---N". These
files have the CLEARONPURGE option set if they have been used to move the extents of the
files set for CLEARONPURGE. For more information on the CLEARONPURGE option, see the
FUP SECURE command in the File Utility Program (FUP) Reference Manual.
•
DCOM has no internal security restrictions except that it requires licensing. The system manager
controls access to DCOM by using the standard disk file security scheme.
•
The version of DCOM must match the version of the NonStop operating system to ensure
compatibility with low-level system interfaces:
Compatible Operating System Versions
DCOM Version
D42 or later
D42
D45 or later
D45
D48.01 or later
D48
G06.06 or later
G06
G06.23 or later
G07
Pre-D42
Pre-D42
•
DCOM needs exclusive access to the file so it does not move the extents of some system files
such as Disk Directory, SMS (storage management system) files, corrupt files, swap files, and
audit trail files.
•
Using the MAXMOVES option, you can perform a fixed amount of compression each day,
depending on the amount of time a system is inactive (when no performance-critical applications
are running).
•
DCOM supports SQL format2 files for SQL/MP.
•
DCOM supports up to 4,500,000 files on a single disk volume.
Guidelines
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