asked on # backup data on dvd using encryption

Linux Back up questions. (Fedora C2)

1) Is there available software to back up data from a raid 1 to DVD using encryption?

2) I read adds from an Internet Backup ISP offering a compression of 10/1 using blowfish algorithm, how can I reach maximum compression and what ratio would that be?

3) Could I obtain the result wanted in 1) and 2) using software from the distro and a script or commands?

4) How can I find the speed of DVD writing (figuring out how long the backups will take)?

Your advices are most welcome.

1) Is there available software to back up data from a raid 1 to DVD using encryption?

2) I read adds from an Internet Backup ISP offering a compression of 10/1 using blowfish algorithm, how can I reach maximum compression and what ratio would that be?

3) Could I obtain the result wanted in 1) and 2) using software from the distro and a script or commands?

4) How can I find the speed of DVD writing (figuring out how long the backups will take)?

Your advices are most welcome.

Storage

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From http://searchsmallbizit.techtarget.com/sDefinition/0,,sid44_gci211759,00.html

single speed [...] is 150kbps

single speed [...] is 150kbps

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Thank you for the detailed explanations.

Compression ratios themselves are completely subject to the data being compressed. No algorithm can guarantee any level of ratio. A 10/1 compression ratio is farsical given real-world data, unless the compressor can guarantee the type of data to be compressed. Let me give you an example: I can guarantee a compression ratio of 100:1 IF you let me veta any data that does not have a single byte repeated at least 200 times.

Alternatively, it can be mathematically proven to be impossible to guarantee anything other than 1:1. If you take a single byte to be the end result of the compressed data, then you can only expand that to represent 2^8 possibilities - full stop. There's no "oh, if you can expand it to that point then you can expand it again...". Nope, 2^8 = 256 different possibilities (i.e., one byte's worth = 1:1). Compression works based on the assumption that the data being compressed can be described in a fashion that can be logically reversed. Not only does that description have an amount of overhead, but a compressed set of bytes cannot be expanded to cover all possible outcomes of data for a given length.

In all cases, it is best to compress data before encrypting it.