Tape Media 20Gb or 40Gb ??

[UniRock]

Hi All,

I have a tape media HP DDS-4 (sticker says 40Gb). I have some confusion regarding its size.
When I insert it and start taking backup (~30Gb), it says "media full" after sometime.
Diskinfo on this says a size of around "20Gb". Why is that so?

I have heard that these can be used for 20 or 40Gb both depending upon compression technology, but not sure how to do it. If that is the case, how can I take backup of 30gb using the same media?

Kindly help.

[Torsten.]

Whenever you see something like 20/40 or 200/400 etc it means native and compressed (assuming 2:1), but this assumption is pure marketing.

If you get 30GB on this 20GB media, you have an excellent compression rate.

[UniRock]

1. So, does that means that diskinfo will show native capacity? (20gb in my case)

A link says:
"actual storage capacity depends on your hardware and software compression ratio"

2. Does above means using fbackup, tar etc will backup different quantity of data on a given tape?

OR

3. Does above means using different device file:
/dev/rmt/c2t0d0BEST
/dev/rmt/c2t0d0BESTb etc

Appreciate your thoughts..

[Torsten.]

You have 20GB raw capacity on tape, but the drive itself will do compression. So it depends if the data is compressible or not (similar to zip or others).

[James R. Ferguson]

Hi:

> Does above means using fbackup, tar etc will backup different quantity of data on a given tape?

The 'fbackup'/'frecover' utilities are designed for high-speed positioning and recovery of files. This is achieved, in part by writing fast-search tape marks. Depending on the number of files, their size and the parameters used during the 'fbackup' sesssion [ as specified by the '-c config' file; see 'fbackup(1M)' ] the utilization of the tape will vary.

The 'BEST' specification in the device file name (or 'b') activates data compression on tape devices which support compression.

Regards!

...JRF...

[Bill Hassell · Accepted solution]

DDS4 can store *ONLY* 20 GB of data on a 20 GB tape. That's all. The tape (and the DDS4 drive) are marked 20/40 but this is very misleading. 20 is the correct number. The tape drive has compression circuitry that can, under certain conditions, store special codes to represent repeating patterns and these patterns are much larger than the codes.

So the amount of compression completely depends on the actual data. Large numbers of text files with fixed records length (very unusual in Unix) can be compressed 2:1, sometimes more. The compression value can be as high as 10:1 or more when the data is just one long string of spaces or zeros, but these files are almost useless except to prove that compression is variable.

So you are guaranteed 20 GB of data, and by using the compression device file (typically /dev/rmt/0m (or 1m or 2m, etc) you might get a bit more than 20 GB. A typical file backup of a standard HP-UX install will see about 1.4:1 compression.

Now diskinfo is useless to use in estimating the size of a backup with the exception of a dd or other raw disk backup program. Otherwise, the command to use is bdf. The "Used" column is what will be written to the tape in a file backup.

When your backup program reports "media full" then you have reached the end of the tape. Do determine if you used the compression device file, use the command:

lssf /dev/rmt/0m

(or whatever tape device you used) The term "best density" means that compression will be used with this device. If all your data files will not fit on the tape, you have several choices:

1. Reduce the selection of files to a smaller list.

2. Run two backups, to split the files between two tapes.

3. Replace the tape drive with a larger capacity drive such as DLT and LTO.

[UniRock]

Thanks all for your valuable comments.

20/40 issue is clear now :-)

[Dennis Handly]

>Bill: when the data is just one long string of spaces or zeros, but these files are almost useless

These files have little entropy, which is equivalent to little information.