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Tape Statistics Explained

Created: 02 Feb 2012 • Updated: 09 Feb 2012
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When you click on the Media tab and then click on a tape media, you will see a screen which looks like this

 

Let's look at some of the more interesting information and what you should do with them.

Creation Date (first column) -

This is the date that this tape is first registered with BE

Allocated Date (first column) -

This is the date that this tape was last overwritten.  The append period is calculated from this date

Modified Date (first column) -

This is the date that this tape was last written to. The overwrite protection period is calculated from this date.

Overwrite protection until (first column) -

This shows the date and time when the tape can be overwritten again.  It is not just the date that is important.  The time is also important.  The tape must be overwriteable BEFORE the job starts if the job wants to overwrite it.

Appendable until (first column) -

This shows the date and time when the tape can be appended to.  A job which wants to append to this tape must start before the end of this period.

Supports H/W Encryption (first column) -

This is important if you want to use hardware encryption.  This is a LTO4 tape, thus it supports hardware encryption.  For other LTO tapes like, LTO3, this property will be shown as No, meaning that it cannot use hardware encryption.  If you need encryption, the you would have to use software encryption for tapes which does not support hardware encryption.

If you want to know more about LTO hardware encryption, read the document linked to in my blog

https://www-secure.symantec.com/connect/blogs/hard...

Hours in use (second column) -

This is the accumulated hours that the tape has been used.

Data (second column) - 

This shows the amount of data that is sent to the tape drive. If you are using software compression, then this number will represent the amount of compressed data sent to the tape drive.  If you are using hardware compression or no compression, then this number is the amount of data sent to the tape drive.

Used capacity (second column) -

The second number is the amount of data that this particular tape can hold.  In this case, 781.5 GB (= 800GB / 1.024) is the amount of data that this LTO4 tape can hold.  For other types of tape, this number will vary depending on the actual capacity of the tape.  Normally, tapes are marketed with two numbers, e.g. LTO4 tapes are marketed as 800GB/1.6TB.  800GB is the actual capacity of the LTO4 tape and 1.6TB represents the data that can be put in the tape if the data can be compressed 2:1.

The first number is the amount of data that is written to the tape at this moment.  Think of this as the used space.

Available capacity (second column) - 

This is the amount of free space on the tape at this moment.

Total capacity (second column) - 

This is the amount of data that this particular tape can hold.  This number is the same as the second number in the Used capacity statistic.

Compression ratio (second column) -

This is the ratio of Data to Used Capacity.  In this case, 720GB / 507.7 = 1.4.  This number indicates how much is the data that is sent to the tape drive has been compressed by the tape drive (hardware compression) before it is written onto the tape.  Typically, the compression ratio is about 1.2 - 1.3.  In case you are wondering, there is no way to improve the compression ratio.  It is a function of the type of data that you are backing up.

If you are using software compression, this statistic will show 1:1.  This is because the Data sent to the tape is already compressed and is equal to the Used Capacity.  To get the software compression ratio, you got to look at the joblog.  There will be a compression ratio for each resource backed up.

It is possible to have a situation where the amount of data sent to tape is less than the used capacity.  That is, the data seems to have expanded.  This is probably caused by compressing data that is already compressed.  For an explanation of this, see my article

https://www-secure.symantec.com/connect/articles/c...

Bytes written and read (second column) -

These are the accumulated amount of bytes written and read since the tape was first registered with BE.

Soft write errors (third column) -

These are write errors that are corrected when the write is repeated.  These could be caused by a glitch in the electronic circuitry, dirty tape head or dirty/faulty tape.  Although these are not serious, a lot of soft write errors would indicate that the tape is failing.  You should replace the tape if the tape has also accumulated a lot of hours in use.

If the tape is new and/or all other tapes have a lot of soft write errors, then it is an inidication that the tape drive is failing or the tape head is dirty.  You should first clean the tape drive with a cleaning tape.  If the write errors still accumulate quickly, then test the tape drive with the manufacturer's diagnostic utility or get the tape drive replaced.

Hard write errors (third column) -

These are errors which cannot be corrected when the write is repeated.  If there are a lot of these errors for a particular tape, then the tape is bad and needs to be replaced.  If there are a lot of these hard errors when a tape is new, then it could be that the tape drive is bad.

Soft read errors (third column) -

The same explanation for soft write errors will apply.

Hard read errors (third column) -

The same explanation for hard write errors will apply.