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How does compression work?

Hi,

I have a 40/80 GB tape drive on my Windows 2003 server. When I perform backups with Veritas Backup Exec v.10 (with open file agent) I get about 48GB worth of backup space on a tape before the next tape is requested. Why can't I get the full 80? Is there a way to increase the amount of information backed up to the tape?

Thanks.
9 Replies

Re: How does compression work?

Hi David,

The figure of 80GB compressed is just an estimation based on assumption that you will get a 2:1 compression ratio. For some reason all the tape manufacturers do this (though I think some use 3:1 as the base), yet this has little relevance to the real world.

In the real world where you and I live, different types of data can be compressed at different amounts, for instance if you took a large text file or .bmp and compressed it with winzip, you would find that it compressed a hell of a lot. In fact a 5MB text file can be compressed to about 400KB!

The compression works by removing all the data which is empty or repeated, and replacing it with a pointer to allow the software to restore it afterwards when required.

Files like .jpeg's, .exe's etc don't generally have much useless data, so don't really compress at all.

So depending on the mix of compressible and uncompressible data on your server, you'll see a lot of difference with the ratio you actually see. From my own experience, and comments from others, I think you can realistically expect to get a compression ratio of about 1.2:1. Anything more than that and you're very lucky.

Re: How does compression work?

The compression ratio comment is on target, but unless you're backing up only files that will not compress, 48 GB out of 80 sounds very low. Even NTFS compressed files usually back up with less space reported on tape than on disk. Since I don't see much in Veritas logs that helps with the question I suggest you go through the ritual of experimenting with different file structures to see what you should expect. Try different tapes also. It's possible to have a faulty tape without having a "bad" tape. One Veritas suggestion on another issue was to long format the tape. If you did this with two tapes and came up with significantly different elapsed times you'd have an indication. And, of course, triple check your BEX settings. I suggest "hardware compression if available, otherwise software".

Re: How does compression work?

Sounds Good. But is there anyway to ensure that I can not get better results? That it is not some other issue? It seems like since most of the files should compress well that the ration of 1.2 to 1 is poor.

Re: How does compression work?

Unless you can talk with someone in very nearly the same environment you are in, only experience will tell. Remember, climate is what you're promised, weather is what you get.

Conversationally I still expect 2:1 compression. In practice I usually get more. Do I get the 1,487:1 compression that one V10 backup reported? I don't hardly think so. Where common MS Office application files are concerned, 2:1 - 4:1 is about right. From the sounds of it your problem might have to do with the specific tape drive and it's driver.

Try testing with no compression, with h/w compression only and with s/w compression only to see what you get.

I'm afraid you're probably alone on the island as you try to figure out precisely where the problem is caused.

Re: How does compression work?

Personally I stand my statement that 1.2:1 is about normal.

One question is what are you actually backing up? Is it a file server, web server, mail server, sql server? Are you backing up the entire server or just the data which is on it?

Remember that if say you're backing up a web server, you will indeed get very good compression of some data, since a lot of it is text based, however, the Windows executables, and the website graphics etc will barely compress at all. Once you put all of that together and average it out, you end up with a reletively low amount of compression.

Re: How does compression work?

Just to add my 2c worth, I've found that BackupExec compression in general is pretty poor.

Files that we were able to get 6:1 compression out of with Winzip only compress to 2:1 with BackupExec.

Ross

Re: How does compression work?

I'll thow my $.02 in here

I generally use 1.3:1 as a rule thumb until I have several backups of a particular server. Each server's ratio will be differnet, but generally is stable in and of itself.

Re: How does compression work?

O.K., another .02

Without drilling down to the individual data types, I'm getting the full rated 2:1 or greater capacity promised by the drive and tape vendors. Over 800 GB on an Ultrium 2 and over 150 GB on a Mammoth 2. That puts my overall compression of MS Office, SQL Server, IIS, system files, software, etc. at over 2:1 compression.

Re: How does compression work?

I guess the answer is no one really knows how much compression to expect. Its different for every drive, file type, and general system setup. I still feel it should be possible to get more compression for the type of files I am backing up.. but that noted... I will try a view things or buy a bigger tape system.

Thanks for all the responses