[Info-vax] backups and compaction or nocompaction might be better

[pcoviello at gmail.com]

On Tuesday, January 29, 2013 10:11:00 AM UTC-5, Stephen Hoffman wrote:
> On 2013-01-29 14:43:53 +0000, pcoviello at gmail.com said:
> 
> 
> 
> > details vms 8.3-1h1  lto4 tapes 8 tape autoloader with encryption. San 
> 
> > clones/copies for backup source. so the tapes raw capacity is 800gb and 
> 
> > below is a chart of what approximately goes to each tape per night, it 
> 
> > does vary. something else that may make a difference is that the 
> 
> > databases that reside on the disks are cache which is fine but they too 
> 
> > are encrypted. I can use as few as 3 tapes and as many as 5.
> 
> 
> 
> Compression[1] and encryption are similar processes of looking for and 
> 
> eliminating patterns in data, but applied toward different goals.
> 
> 
> 
> Always compress your data first, then encrypt it.
> 
> 
> 
> Don't bother trying to compress encrypted data.  Properly encrypted 
> 
> data should never[2] be compressible.  (In practice, I've encountered 
> 
> cases where attempts to compress encrypted data produced larger outputs 
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> than what was input, too.)
> 
> 
> 
> There's no point in encrypting already-encrypted data, either.  That's 
> 
> CPU intensive, and for no gain.
> 
> 
> 
> Run your tests with just compression (either at VMS or at the device, 
> 
> but not both) enabled, and see what you get for data volumes.
> 
> 
> 
> Then turn on encryption of the compressed data, and test again.
> 
> 
> 
> I'll presume you know that BACKUP /IGNORE=INTERLOCK is a bad backup; 
> 
> that both overt and silent corruptions are permissible within the 
> 
> created saveset.
> 
> 
> 
> ————
> 
> [1]VMS calls data compression compaction, when it's performed at the 
> 
> device level.
> 
> [2]If the encrypted data is compressable, then the encryption is badly broken.
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> 
> 
> 
> 
> -- 
> 
> Pure Personal Opinion | HoffmanLabs LLC

that was what I was afraid of and unfortunately I can not un-encrypt the databases :-(   

as far as the interlock I thought I had seen something on that awhile back and totally forgot about it.

thanks 
Paul