Backing up data to a DDS3 Tape drive

[Alexander Laing]

Hello,

I have an HP C1537A tape drive, and I use DDS3 tapes.

Each day I backup a set of files by first creating a "tar" archive that results in the "tar" file being about 1 gig.

I find that I can only backup around 12 of these onto a single tape.

But with another situation I have a directory on the UNIX server "/backup/redo_archive" that stores all of the ORACLE "redo" logfiles. There are a total of 20gig of files in this directory of which all of them can be backed up to a single tape using the command

"tar -cvf /dev/rmt/0mn /backup/redo_archive/*"

Can anyone explain why I can only backup 12gig of the previous "tarred" files but up to 20gig of "untarred" files.

Thanks.

[Sanjay Kumar Suri]

The capacity of DDS3 tapes seems to be 12GB which explains why in the first place only 12 1GB files are backed up.

tar does not do file compression as your command also shows.

Are all archive redo log files of the same size? In case "alter system switch log file" kind of command is used, the resultant redo/archive log files can be smaller.

sks

[john korterman]

Hi,
are you sure that the tape holds 20gb of archived redolog files and do you use the same device file in both situations?
If you do not use the same device file the long shot guess is that /dev/rmt/0mn uses hardware compression, although that is rather unlikely.

regards,
John K.

[Senthil Kumar .A_1]

hello alexander,

first make sure u can only backup 12 files of 1 GB file,that is if u can backup a redo log file of 20 GB on the same dat drive i guess u can as well store ur tar output's exeeding 12 GB.


the reason for redo log files (20 GB) that can be stored is,because of the fact that DDS3 technology supports 12/24 GB,12 uncompressed and 24 GB compressed.whether u are storing tar output in the compressed mode or uncompressed mode fully depends on which device file for the tape drive u are using while backup.the minor number of the device file decides the mode in which that device is accessed .U can get all the device file's used for ur tape drive using "ioscan -funC tape" command.Then use "lssf" command on each device file.the device file in the format /dev/rmt/cxtxdxBEST would utilize the best density supported by the device,for example 24GB in compressed mode in ur case,If u use /dev/rmt/cxdxtxDDS then possibly it will write in 12GB uncompressed mode in ur case.If u are using /dev/rmt/0m or 1m or 2m....then ll on this device to see whether these are linked to cxtxdxBEST(this is usually the case in HP-UX 11.x machine).if these device files are not links perform "lssf" on these files to find out in which mode it will write.

as on final note i guess if u can take the backup of 20GB redolog file, then u can as well backup 24 1GB tar archive on the same device.

Please test it & give me the feedback....

[Patrick Wallek]

The amount of data you can get on the tape depends ENTIRELY on the compressability of the data. The tape drive will automatically compress the data.

The 24GB compressed that the DDS3 can handle assumes that you will be able to compress ALL files so that they are 1/2 of their original size (a 2:1 compression ratio). That depends entirely on the data. Regular ASCII files generally compress VERY WELL (sometimes 10:1 ratio) but things like pictures (JPG files) and movies do NOT compress well at all. If you try to compress something that is already compressed you can wind up with a larger file.

Database files and redo logs can compress fairly well as there is generally a lot of empty space in them so I am not surprised that you can get 20GB of these on a tape. It is possible that you could get even more than that on a tape.

Now your tar files will depend on the data that is inside the tar file. Just because it is a tar file does not mean that you will or won't get more than 12GB on a tape. Have the tar files been compressed or gzip'ed before you put them on the tape? If so, then I'm not surprised at the 12GB you get. If not, then you need to have a look at the data in the tar files and see how compressable it is. You might even try running gzip or compress against one of the tar files and see how much it shrinks. If it doesn't shrink much then your data is not very compressable. Your tape drive will generally not compress any better than compress or gzip.

I hope this helps to explain it and didn't muddy the water more for you.

[A. Clay Stephenson]

Wise admins will rely solely upon the native capacity of any media. Patrick's explanation is exactly on target. You should plan your backups around the native 12GB capacity; that way there are absolutely no surprises.

[Michael Schulte zur Sur]

Hi Alex,

it would be necessary to know, what kind of data the pretarred files are and if the procedure, that tars them, does not also compress then. That would explain, why you get only 12 of them on a tape. Otherwise, the pre tarring does not save that amount of space.

Michael

[Alexander Laing]

Thanks for all of your responses.

It appears that Patrick did hit the nail on the nead.

The "pretarrred" files were already "gzipped" by the same procedure that "tarred" them. Therefore they cannot be compressed any further.

What I will do is adjust my backup strategy to have the tape drive's built-in compression act on the database files while they are being archived to tape.

Once again thanks!