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Question about tapes

Bruno_Villeneuv
Level 4

Hi guys,

been searching for a while regarding this problem. Let me do my best to explain:

I have BE 2010 R2 on Win2003R2 x86 server. The setup which was already done here consists in 4 backup jobs:

1- Backup Mon-Wed

2- Backup Tue-Thu

3- Backup Weekend

4- Backup Exchange

 

I'm not even sure why it's been seperated in two jobs (Monday through Thursday) because the selections are the same .. so i guess i could put those two jobs in a single one but change the period for every week days but friday (backup weekend). Anyway.. I'm using 800Gb RW Ultrium3 LTO tapes for the record.

The thing is that the backup, lets say Mon-Wed, starts correctly at 7:00pm. It backs about 538Gb up of data. After that, Exchange is scheduled to start at about 1:00am. The job starts, backup for about 112Gb then waits for another tape. 538+112=650Gb of data. Am I missing something or my tape is 800Gb? What is this 800Gb? Is it 800 before it is compressed or after?

If i go in media, the compression rate of my Monday-1 tape is: 1.6:1 (What does that means?: Take 800gb*1.6 for being able to backup 1280Gb ?)
The compression rate for the second tape i put in (is a 400Gb tape): 1.9:1

I would like to be able to change the compression rate of my tapes also .. because they all look like they've been configured differently or by different persons. (Got 1.3, 1.4, 1.5, a bunch of 1.6's, etc)

Thank you for your help.

1 ACCEPTED SOLUTION

Accepted Solutions

pkh
Moderator
Moderator
   VIP    Certified

LTO3 tapes have a 400GB uncompressed capacity.  This means that if the data is not compressed.  You can only put 400GB onto 1 tape.  This uncompressed capacity is shown as 390GB in your screenshot.  This reduction is due to overheads, etc.  All compession ratio are measured from this uncompressed capacity.  If you get a compression ratio of 1.5:1, you managed to get 600GB onto your tape.  Normally, the compression ratio in real life is around 1.2 - 1.3, so from your screenshot, you are getting better than average compression ratio.

Forget about 800GB, this assumes that you can compress your data to half their size so you are able to put 800GB onto a 400GB tape.  In this case, the compression ratio which is the ratio of compressed to uncompressed data is 2:1.  This is hardly achievable in real life, so forget about it.

When you compress data, how much compression you can get depends on the type of data.  See my article below for an explanation.

https://www-secure.symantec.com/connect/articles/compression-short-explanation

This is why in your screenshot you can see different compression ratios.

When a tape is full, BE will prompt for another tape.  This is normal.  If you are compressing your data, when the tape is full depends on the compression ratio.  It might be full when you put 560GB onto the tape or it can be full when you put 450GB onto the tape.  This depends on the type of data that you are backing up and how compressible they are.

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7 REPLIES 7

Kiran_Bandi
Level 6
Partner Accredited

LTO 3 media is of capacity 400 GB (native) and 800 GB in compressed mode. In general very rarely one can see a media holding data equal to it's compressed capacity. You can consider 1.9:1 as very good compression ratio. 

1.4:1 can be considered as good compression ratio in general.

What does that means?: Take 800gb*1.6 for being able to backup 1280Gb ?)

You have to calculate this against native capacity i.e 400GB*1.6 = 640 GB.

Bruno_Villeneuv
Level 4

So am I correct stating that a 800Gb tape compressed at 1.9:1 is gonna able to handle over 1.5Tb of data? Or it's the opposite? 

My Wednesday-1 tape, as the picture above, looks like 390.9Gb in size. Compression rate is 1.6:1 so i guess it's taking half the capacity instead of increasing it. I mean, tape is 800Gb but as the compression one and a half would mean, 1/2 the size of the tape. True capacity is 400Gb then ... That's why BE asks another tape but the job seems to be able to finish (over 530Gb)... i'm missing something.

Kiran_Bandi
Level 6
Partner Accredited

Your tapes are of capacity 400 GB. It can hold upto 800 GB of data if the job achieves 2:1 compression ratio. 

Wiki page explaining LTO: http://en.wikipedia.org/wiki/Linear_Tape-Open See the table explaining capacities of LTO 3. 

pkh
Moderator
Moderator
   VIP    Certified

LTO3 tapes have a 400GB uncompressed capacity.  This means that if the data is not compressed.  You can only put 400GB onto 1 tape.  This uncompressed capacity is shown as 390GB in your screenshot.  This reduction is due to overheads, etc.  All compession ratio are measured from this uncompressed capacity.  If you get a compression ratio of 1.5:1, you managed to get 600GB onto your tape.  Normally, the compression ratio in real life is around 1.2 - 1.3, so from your screenshot, you are getting better than average compression ratio.

Forget about 800GB, this assumes that you can compress your data to half their size so you are able to put 800GB onto a 400GB tape.  In this case, the compression ratio which is the ratio of compressed to uncompressed data is 2:1.  This is hardly achievable in real life, so forget about it.

When you compress data, how much compression you can get depends on the type of data.  See my article below for an explanation.

https://www-secure.symantec.com/connect/articles/compression-short-explanation

This is why in your screenshot you can see different compression ratios.

When a tape is full, BE will prompt for another tape.  This is normal.  If you are compressing your data, when the tape is full depends on the compression ratio.  It might be full when you put 560GB onto the tape or it can be full when you put 450GB onto the tape.  This depends on the type of data that you are backing up and how compressible they are.

Bruno_Villeneuv
Level 4

Thank you for these precisions. That helps me understanding this topic. This is kinda complicated to get but i'll read your article.

Last question, once the tape has been compressed once, is it possible to change the compression ratio? I should probably be doing some research on some other threads but i guess you know the answer.

 

Thanks again.

Kiran_Bandi
Level 6
Partner Accredited

once the tape has been compressed once, is it possible to change the compression ratio?

It is not the tape but the data which will get compressed. As PKH already mentioned, compression ratio very much depends on the type of data. We can just enable or disable the compression on backup job. But can't change the compression ratio.

Bruno_Villeneuv
Level 4

I just finished reading the article and I should've wait before replying this post :)

Indeed, depending on which data is being backed up, the compression ratio changes. Thanks.