Media Usage...no way this is correct?

[Rick_Flair]

Hi all,

I'm presently trying to calculate our business' tape requirements by following the "Calculate media needed for full and incremental backups" section of the Backup Planning and Performance Tuning Guide.  In short, it recommends the following equation:

 Number of tapes = ((Amount of data to back up) * (Frequency of backups) * (Retention period)) / (Tape capacity) 

	Data	Frequency/month	Retention (months)	GB	TB
Full	15320	4	12	735360	718.1
Incremental	4689	20	0.5	46890	45.8
Total data tracked =				782250	763.9

According to our table above, given we're using LTO3 tapes without compression, we're looking at quite large media requirements....

	Data (GB)		Tape Capacity (GB)		Tapes
Full backups	735360	%	400	=	1836.6
Incremental backups	46890	%	400	=	116.9

Total tapes required = 1953.5

Does that look right??  Can someone talk me off the ledge?

[Marianne · Accepted solution]

That's the absolute minimum number of tapes....

For one, there are at least 4 months in a year that have 5 weeks... And about 22 working days in a month.

These calculations are also assuming that all tapes will be filled - if tapes are taken out of the robot on a regular basis, chances are that they will not be full. If you have more than one media server without media sharing, more tapes will be used. If you use multiple pools, you will need more tapes.

Then there's bad media every now and then or one stream of a multiplexed backup that fails - this space cannot be reclaimed...

Enough bad new - if you are lucky (like most of us!) and get good compression on tapes, you will probably see about 600Gb of data written to tapes.

This calculation is good motivation to your management to purchase enough tapes.

[Andy_Welburn]

We have some LTO3s that have over 1Tb data stored on them as far as NB is concerned.

TOTAL 430Tb stored over 766 tapes ~ 570Gb/tape

of which FULL tapes 400Tb stored over 650 tapes ~ 615Gb/tape

[riva11]

I agree with previous comments, also another point to consider is maintain some tapes in case of bad LTOs or tapes with many soft/hard errors.

[Andy_Welburn]

the thing to bear in mind is that you'll not need ALL of these tapes from the outset - you'll certainly need enough to get you through the first few months at least which will give you a better indication of your usage. Luckily these days tape failures are minimal & will not amount to much in the grand scheme of things.

From what I can see from our records, in the 3 years we have been using LTO3 tapes, of our current total (say 900) 300 were bought in the first 2-3 months, another 300 over the next year, 150 the following year and 100 this year. Out of this we may have lost less than a dozen due to failures of one kind or another.

[teiva-boy]

Perhaps you can eliminate some of the tape usage and associated tape management by backing up to disk (possible play for dedupe here) and replicate the data?

At the very least, the incrementals could go to disk, that would eliminate more than a hundred tapes and the tracking and management of those tapes.  

If you had deduplication licensed, and two media servers, you could do an optimized duplication of the backups to another location, which would give you some piece of mind in having a redundant copy since there is no tape.

As you get more comfortable with the technology, perhaps you'll have a month or more of backups on disk, and only export out monthly tape sets, and not weekly ones, which could again eliminate much of the tape management.

Tape is still MUCH cheaper from a capex perspective than disk and disk+Dedupe.  But the OpEx saved and management time saved from not having to manage tapes is priceless!

[J_H_Is_gone]

I use LTO4 and have smaller retention then you  and I have about 1300 tapes.

Playing with some settings I try to FILL as many tapes as I can and still get the backups finished within my window. but to do that not all the tapes are full when I remove them from the robot.  So yes you need a lot of tapes.

[Rick_Flair]

Thanks all for your quick replies!  All good responses....Marianne van den Berg answered first, so I'm marking her as the solution...