DDS3 and compression

[Kim Kendall]

I would like to tar a directory onto a DDS3. The du command says it's a little over 12GB. All the contents of the directory have already been gzipped.

First, I tried tar to the 0m device but hit the end of tape error. Then tried using the gnu tar with -cfz switch, but get a
"Cannot write : File too large" followed by an unrecoverable error message.

The DDS 3 is supposed to take 24 GB (compressed). The question is.... can I take 12.x GB of data that's already compressed and write it to a DDS3?

[Leif Halvarsson_2]

Hi,
Don,t use hardware compression if data is already compressed, in some cases this can increase the data written to the tape. Try with uncompressing the data first, or use a uncompressed device file.

[harry d brown jr]

That's a close one. Usually a compressed file, when compressed again will EXPAND in size. Yes, it gets bigger.

Have you tried cpio ?

Either way, it'll probably take two tapes.

live free or die
harry

[Kim Kendall]

Leif,

uncompressed device file?

[Kim Kendall]

Never used the /dev/rmt/c0t0d0DDS[b,n,nb] device files. They have different inodes from the other devices. What are these?

[Patrick Wallek]

Harry is right. Since the data is already compressed, compression on the tape drive will probably not do you much good.

A DDS3 is advertised as 12 GB native and 24 with compression (assuming 2:1, which may or may not happen depending on the data).

I would say that you would be better off, if you have room in your LVOL, gunzip'ing all of the files, and then writing them to the DDS3 drive and see if you can get them all there.

Another option is to gunzip and re-gzip them with the option '-9' or '--best' option. This is slower, but better, compression. If you compress like this, maybe you can get it down to where you can get it all on a tape.

[Bill Hassell]

12Gb is all that the tape will handle. If the data cannot be compressed (which is the case with already compressed data), then you're probably just over the limit. Change your backup to 11Gb and it will probably fit. And of course, the data won't fit is there a a few bad spots on the tape (the drive erases the bad areas and skips over them), or the tape slightly shorter than normal.

If data is already compressed on disk (via software), hardware compression will have no effect, or as mentioned, possibly increase the data size slightly.

tar cannot handle any file larger than 2Gb, GNU tar cannot handle large files either unless it was specifically compiled for large file support. Since this makes the tar format incompatible with classic tar programs, you can only restore the data with the same version of GNU tar.

[Leif Halvarsson_2]

Hi
Creating an uncompressed device file for DDS3:
First get the hardware path of your drive:

ioscan -C tape

Create the device file (replace the hardware path in my example with what you get from ioscan).

mksf -b DDS3 -H 0/0/2/0.0.0

This creates a file in the /dev/rmt directory ending with "DDS3".

An alternative it to use the switches on the drive to switch off compression. Use this link:

http://www.hp.com/cposupport/information_storage/support_doc/lpg51068.html