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The client COMPRESSION option is only in effect if your Tivoli Storage Manager
administrator specifies that your client node may choose its own compression.
When a node is registered using the register node command, there is a
COMPRESSION parameter that allows the server to enforce compression,
enforce non-compression, or allow the client to choose for itself. The default
value is that the client can choose by setting the COMPRESSION value in the
client options file. If the administrator wants to force a client to use compression,
regardless of the contents of the client options file, then use the command update
node with the parameter compression=yes.
General rules for using Tivoli Storage Manager compression:
If you are backing up directly to tape, and the tape drive supports its own
compaction, use the tape drive compaction, not Tivoli Storage Manager
compression. If you have already compressed the data at the client level,
then the tape drive compaction will not be able to reduce the data further.
Do not use Tivoli Storage Manager compression if a client currently has
built-in file compression support, like the Tivoli Storage Manager client for
NetWare. Tivoli Storage Manager compression on these clients will not yield
additional reduction in the amount of data backed up to the server.
If you are using compressed file systems (for example, NTFS compression on
a Windows system), the Tivoli Storage Manager client reads the
uncompressed data stream regardless of the compression state of that file in
the file system. This means that the file is uncompressed during the read and,
if it is compressable data, gets compressed again by the client during the
backup. This decompression/compression overhead may decrease backup
performance. When the same the file is restored, the client will decompress
the file data received from the server and write the uncompressed data to the
file system, where it will get re-compressed by the file system driver. Because
of this extra processing required, we do not recommend using Tivoli Storage
Manager compression on NTFS compressed file systems.
COMPRESSALWAYS
The COMPRESSALWAYS option controls what occurs when a file grows during
compression. You can continue compressing, or send the object again as is if it
grows during compression. This option is used with the COMPRESSION option.
DIRMC
This option had a big impact on restore performance in earlier versions of Tivoli
Storage Manager, where directories were restored first, followed by the files. It
was therefore an advantage to have directory information cached so that it was
permanently on disk, and best practices design was to create small storage
pools specifically for directories. The DIRMC directive in the client options file
was used to bind directories to a management class pointing to this storage pool.
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IBM Tivoli Storage Manager Implementation Guide
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