D Wood Posted August 20, 2003 Report Share Posted August 20, 2003 Is compression during backup automatic, or is there somewhere to turn it on? During my first backup, the preview says it will back up 6GB, and needs 9 700MB CD's to do it - is that right? It doesn't sound like it is compressing the data - my old version used to have a footnote on the menu showing what % compression was being used. I'm running Retrospect 5.1 on a 17" iMac, OS 10.2.6. Link to comment Share on other sites More sharing options...
natew Posted August 21, 2003 Report Share Posted August 21, 2003 Hi Compression needs to be turned on in the Options of your script or immediate backup operation. Nate Link to comment Share on other sites More sharing options...
Slaroye Posted August 29, 2003 Report Share Posted August 29, 2003 I am having terrible compression. I am running Retrospect 5.1 on a G4 DP 1.42 w/ 1.5 gig RAM, ATTO Ultra160 card and a LaCie AIT Autoloader (Sony mechanism). The LaCie has 4x 35 gig tapes (claims 91 gig compressed!) and after backing up a TiBook with 8.1 gig, another TiBook with 5.2 gig, an Xserve with 5.2 gig and an Xserve RAID with 159.7 gig - total of 178.2 gig - all the tapes were FULL!! I should get 140 gig uncompressed and up to (dreaming) 364 gig compressed. The drive has 'DC' on the display meaning hardware compression is enabled. I have run the script with software compression on and off with no difference (the compression is set to ~Compression Filter when on). Why can I not fit more on these tapes? Link to comment Share on other sites More sharing options...
Slaroye Posted August 29, 2003 Report Share Posted August 29, 2003 Sorry - I have now updated the Client machines with the latest version of the client software and will try to run the scripts again to see whether that might have an influence. Link to comment Share on other sites More sharing options...
mcswgn Posted August 29, 2003 Report Share Posted August 29, 2003 There are a LOT of things that affect compression. First thing to remember: Drive manufacturers LIE. The standard line used to be to claim 2x compression (DDS3 tapes are 12GB native and "24GB" compressed). I notice that AIT tapes are now claiming 2.6x compression. Did they really change the compression algorithms that much? I don't think so! They have been tweaking compression algorithms for a LONG time, and there just isn't that much space left to squeeze out of files. This smells like a number dreamed up by a marketing department! Second point: Even if the compression ratio was accurate, it is based on some mythical "average" collection of files that is heavily skewed to text documents. Binary files (system files, pictures, PDF files, etc.--in other words, probably 90% of the typical disk's contents) won't compress AT ALL. In fact, they usually get LARGER when "compressed". In real life I see anywhere from 1.2x to 1.5x compression to my DDS3 drive (based on looking how much actually gets put on one 12GB tape), where I'm backing up full machines, not just documents directories. (However, these are mostly from slow clients, so see point three below.) Third: Compression is optimal only when data is streaming continuously to the drive. If the data is not coming in fast enough to keep the tape going, then you will lose capacity. I'm guessing that yours is an AIT or AIT+ drive, based on the 35GB native tapes, which isn't as fast as AIT2 or AIT3, but still pretty fast. However fast the drive is, you need data to be coming in at that rate AFTER compression. So if you really got 2x compression, you would need data to come it at TWICE the drive's transfer speed. If the data is slower, then the drive will typically write nulls to the tape while it waits for more data. (It's very bad for the drive and tape to constantly be starting and stopping, so most drives will try to avoid this.) Clearly that won't do much for the overall capacity of a tape! If you really backed up 180GB to 4-35GB tapes, you got a compression ratio of 180/140 or about 1.3x. This may be as good as it gets, but if you want to try to do better, look to point three above. (You probably actually backed up less than 180GB, because Retrospect will do matching and not write the same file more than once. If there were any duplicate system files, for example, among the four machines, only one copy of each would be written. However, with the lion's share of the data coming from your Xserve RAID, you may not have had that much duplication.) Link to comment Share on other sites More sharing options...
Slaroye Posted September 4, 2003 Report Share Posted September 4, 2003 Thank you very much for your reply!! I think you are exactly right with point 3. I have found lately that all machines on our network have slowed down even though all desktops are connected to the Xserve through a 10/100T switch and my machine (running Retrospect) connected though a 10/100/1000T switch to the Xserve. I will look into this further... Link to comment Share on other sites More sharing options...
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