Compaq DAT 20/40

[Robin Collins]

Hi Scott

To compress the folder in Win2K, right click on the folder, go to properties, advanced, and select 'Compress contents to save space' - this can be helpful with some file types.

I haven't been looking too hard at compression on my system, but a quick check shows 1.42 to 1.54 just using hardware. If you got 28 gb onto a 20/40 tape then you've got 1.4 compression which is exactly what I'd expect.

The compression you are getting will show on the Activity Log tab as a line saying Media Compression Ratio n:nn.

As regards costs, you'll probably find that adding a 160gb IDE drive is a whole lot cheaper than a new DAT drive and lots of new tapes - depends on how you work and time pressures but worth a thought.

Although I do know a lot of folks use 'em autoloaders can be a bit cranky and a number of comments on the old Arcserve forum suggested that you need the really expensive top end jobs for reliability.

Hope this helps anyway

Robin

[Dave Dewar]

Hi,

The tapes you are using are DDS4 20/40GB tapes. As Robin says 28GB is a compression ratio of 1.4 which is what I would expect on typical file system data. Hardware compression is working and it is giving you expected compression ratios.

As I mentioned previously, you could try using software compression instead. The algorithm used by windows will typically give you a bit better compression ratio but you will eat CPU cycles if you are doing it inband (during the backup with Arcserve). Your other option is to create a disk image of you backup job on your backup server and compress this before you start the backup. Then send this down to your tape drive with hardware compression disabled. You will probably get about 1.5-1.8 compression ratio doing this.

I would also agree with Robin that a large IDE drive will be cheaper than another tape drive or an autoloader.

Robin's comment about the dat autoloader reliability is justified to some extent. The reason for this is that some customers use this product as a large tape drive and forget the fact that the duty cycle spec of these products are the same as standalone DDS drives, 20% or less. The product should really be used as an unattended backup device that uses at most two tapes per daily session.

Cheers,

Dave Dewar

[Scott_186]

Thanks again you two.. good info!

But my question remains: Why is it getting to 28 gig written to the tape and then asks for a new BLANK tape? it it's only got 28 gig on there, why is it asking for a new tape? Is it really full or do I still have a problem in there somewhere?

And, How come you say I should buy a new IDE drive? Are you saying copy the data to that as opposed to writing it to tape?? That's probably a stupid question, but I'm just a litt confused on why I'd buy another hdd.

Scott

[Dave Dewar]

Hi Scott,

Ok we have a fundamental misunderstanding about compression here, maybe :-)

The native capacity of a DDS4 tape is 20GB, thats 20 real true GB. The tape will not hold more than 20GB.
When we say the tape can hold say upto 40GB of compressed data we are saying that if we take 40GB of data, say a single large file and compress the file before we put it on tape then with a good compression algorthm and compressible data we might be able to actually remove redundancy from our file and end up with an compressed file that is smaller in size, maybe even as small as 20GB. Hence, we then put this on tape. Winzip is a claasic compression program. Zip up the file and it reduces in size if it contains redudant info. Try zipping a jpeg that is a compressed picture format and there is no redundancy to remove and so the file doesn't reduce in size.

When we do hardware compression in a tape drive the compression occurs before we actually write the data to tape. We take it from the host, store it in a memory buffer, compress it in reasonable sized chunks and write to tape. Uncompressing obviously works in reverse.

Hence, your tape drive is taking 28GB in, compressing it down to about 20GB and writing it to tape. The tape is then full and Arcserve asks you for another tape to complete your backup job. This is called tape spanning and most backup applications support this. Label your second tape "2 of 2" and insert it and Arcserve should complete the backup and your backup image will span across two tapes. You will need both to restore.

Apologies if most of the above is blinding obvious to you already, its sometimes difficult to gauge how much people already know from forums :-)

As to your second question, you don't have to spend any more money at all if you don't want to.
If you don't mind doing tape spanning and getting your 5 servers backed up on two tapes and you are happy with the 2.5-3MB/s you are achieving, then you are all set.
If you want to get your backup on one tape then you need a tape drive that can handle more native capacity, say a DAT72 that has a native tape cap of 36GB or some even bigger.
If you want to get better performance, then I think Robins suggestion of doing an image backup on a new IDE disk on your tape server and maybe partitioning that up to give you several volumes to image to, and then following that up with an backup of the image to your tape drive every so often might give you some performance benefit.

Cheers,

Dave Dewar

[Scott_186]

Thank you guys so much for your help on this..

Cheers..

The only thing left that I need to do is buy a new drive with a large native capacity.. Plain and simple..

Scott

I feel like I should have a certification or something out of this, it's been a very knowlegable couple weeks!!