Networker

[Networker] How to tell how much physical tape is being used?

2007-04-24 15:36:17
Subject: [Networker] How to tell how much physical tape is being used?
From: George Sinclair <George.Sinclair AT NOAA DOT GOV>
To: NETWORKER AT LISTSERV.TEMPLE DOT EDU
Date: Tue, 24 Apr 2007 15:35:39 -0400
Is there a way to determine how much physical tape space is being used by a backup?

For example, lets suppose I run an Oracle backup using RMAN with NMO, and the RMAN script uses Oracle's compression feature (compressed), and let's say the backup is a level full and creates 5 save sets at around 200-300 MB each (each save set has say 4-6 files each). Next, I run the same backup, but this time I don't use the 'compressed' option in the RMAN script, and maybe I get 5 save sets again, but this time they're much larger, say 1 GB each. I recover everything in both examples, and the recovered database is exactly the same as far as the number of files and their sizes. Well, hardware compression is enabled on the drives, so the data is getting compressed both times, and in the second example, while I may be backing up approx. 5 GB of data, I'm probably not using 5 GB worth of tape space due to hardware compression, so it is possible that both examples result in the same amount of physical tape space being used? Maybe?

Is there a way to tell which method is actually using more tape or if they're close?

I can query for the 'mediafile' numbers for the save sets, and I notice that they always start 2 beyond the previous one, but a given saveset could be of any size so this doesn't really tell me how much tape is being used, right?

Here's why I ask such a goofy (stupid?) question. OK, we ran this Oracle backup (level full) with the 'compressed' option, and we noticed that the backup size was very small (kinda nice!), but as a result, the drive speeds were very slow. This is not surprising given that we probably were underfeeding the drives since the client was sending less data due to the Oracle compression. Next, we run the same thing but without the 'compressed' option, and now the drives scream (again, not surprising given that the client now has more data to send), but the backup sizes were way larger. Overall, however, the backup completion times were reasonably similar. I'm wondering if maybe in the second example that because the data was not already compressed that the drives shrunk it down and that maybe it's not really taking up any more space on the tape than the same data in the first run. We recovered both sets, and the total number of recovered database files and their sizes are the same in both cases. How can I determine, therefore, which method used less tape?

Thanks.

George

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George Sinclair - NOAA/NESDIS/National Oceanographic Data Center
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