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Disable/Enable Hardware Compression When Using BRU

Views: 16864
Votes: 3
Posted: 18 Nov, 2008
We get the question of tape drive hardware compression disabling a lot, but it's answered by understanding the way that tape drives work.

All current tape drive vendors use some version or another of an adaptive compression algorithm.   What this means is that the data stream coming in is monitored for the results of compression attempts and when the data is non-compressible, the compression logic passes the data straight through untouched.

This, combined with the fact that the drives use a dedicated chip to handle the compression, means that the drives will handle the incoming data stream at the best possible speeds regardless of the compression state.   Plus, because of the dedicated compression hardware, the drive's throughput is dependent on the data compression being enabled.  In fact, if you disable the hardware compression you can actually slow down some drives compared to letting the compression engine do its "thing".

Finally, with applications like BRU where there is compressible data in the I/O stream because of our headers and the metadata, there is always "some" level of compression occurring - even when the actual data is already compressed.

All of that leads us to the conclusion that allowing the drive to decide is the best way to operate with disk.

Hopefully that helps you to understand why we don't offer a switch for the hardware compression.
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