Inquiry on tar command

[Mei Jiao]

If I extract only a file from a tar backup tape, would the tar process stops and rewind the tape after that particular file is being restored? Or will it still reading through the end of the tape and only rewind by then?

Is there any way that we could do to stop the tar process continue to read through end of the tape and start rewinding after that particular file is being restored?

Please advice, thanks!

[Steven E. Protter]

What happens depends on what command line options you use.

What device you use controls the behavior, you can choose a device /dev/rmt/ etetera that will not rewind at all.

SEP

[Mei Jiao]

Hi SEP,

Thanks for your reply. However, I have gone through the man page of 'tar', but did not find any option if I want to do this:
- Stop the tar process once it restored the particular file I want to restore, and start rewinding back, instead of it reading through the end of the tape.

I used the normal device file: /dev/rmt/0m

Pls advice again. Thank you!

[Nguyen Anh Tien]

Hi Mei.
Happy new year.
I think tar can not extract only a file. Check attached file for detail.
You should use cpio command for your purpose.
tienna

[Mei Jiao]

Hi Tienna,

Well I think 'tar' can restore a particular file.

[abhijeet_7]

Hi Mei Jiao,
I think tar can extract a single file..
lets try this (not much sure!!)

tar -xvf /dev/rmt/0m /homt/mel/

It will read till end of tape & rewind from there.

let us know

regards
ABHI K

[Mei Jiao]

Yes, I've done a testing myself yesterday. The 'tar' command will read through the end of the whole backup before it rewinds.

I've also tested on fbackup, after frecover restored the file it comes to the prompt much faster than 'tar'.

Thanks!

[Ravi_8]

Hi,

tar, once find the required file on the tape and extracted it, it runs through the tape to find another such file in the tape if exist( in case of incremental backup the chnages will be extracted). Hence the reason tar will read through end of tape.

If the tape size is bigger (eg. 120GB ...)this process takes longer time. Hence the concept of (NAS) network attached storage. Wherein NAS maintains the database of backedup file, while restoring it check the database for the files to be restored and point to particular tape in the tape library and moves the header to particular block in the tape and read/extract it.

[Thierry Poels_1]

hi,

as you can update (read "append") a tar backup, I guess it's quite normal that tar reads the whole backup file to check for newer versions of the same file.

regards,
Thierry.