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Configuration
Small capacity or slow response storage devices
Compression takes time, so you need to decide whether the compression is
helping or hurting the total elapsed time of your operation. In general, very high
speed networks do not benefit as much from compression because network
bandwidth is usually not saturated. Conversely, older client CPU models may run
slowly because the CPU cannot compress data fast enough to keep the network
connection busy.
Many sites have various combinations of these configurations, such as slow
networks, slow CPUs, and small/slow devices. In this case, make a judgement
call about using Tivoli Storage Manager compression. Performing tests may
show you the most efficient mode for your environment.
Data compresses to varying degrees depending on the content of the files. Data
composed of text, or many repeated characters like blanks, compresses well.
Data that is already compressed, and data consisting of random characters like
executables, does not compress well, and may actually grow.
If you decide to use compression, enter your data reduction rate. Use Table 2-4
to estimate the data reduction if your actual ratio is unknown, but the best advice
is to make some tests on your actual data.
If you decide not to use compression, or are unsure whether to use compression,
enter one (no compression) in the Data Compression field.
If you do not use client compression and your tape device has a hardware
compression capability, then the data will still end up compressed when it
reaches a tape volume. If you do use client compression, then you should disable
any hardware compression capability on your tape devices, since data in general
cannot be
Table 2-4 Typical data compression ratios
Database data
Print and file server data
Executables, compressed data,
encrypted data
22
IBM Tivoli Storage Manager Implementation Guide
doubly compressed
.
Compression ratio
Data reduction
3:1–4:1
0.66–0.75
2:1
0.5
1:1
1
Compression
Yes
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